Adjuvants

Substances added to vaccines to strengthen the body's protective immune response to the vaccine.

Adverse event

Any kind of symptom or health event experienced after vaccination. Not all adverse events are caused by the vaccination; some may be coincidental.

Anaphylaxis

A severe allergic reaction of sudden onset and rapid progression, usually accompanied by hives and/or flushing of the skin. It affects two or more organ systems at once and can lead to difficulty breathing, feeling dizzy and/or abdominal pain with vomiting.⁵

Antibodies

Proteins made by cells of the immune system that can identify microorganisms like bacteria and viruses and prevent them from infecting cells.

Antigens

The parts of pathogens or their toxins that are used in vaccines to provoke an immune response.

Bacteria

Single celled organisms (living things) that exist in our body and our environment. Most bacteria are harmless and some are beneficial to humans; however, some bacteria can cause disease.

Herd immunity

Occurs when a significant proportion of individuals within a population are protected against a disease through immunisation. This offers indirect protection for people who are still susceptible to the disease, by making it less likely that they will come into contact with someone who is carrying the pathogen. Find out more about herd immunity

Immunisation

The process through which people are protected against illness caused by infection with pathogens.

Immunity

The state of protection that occurs when a person has been vaccinated or has had an infection and recovered. Vaccination, like infection, confers immunity by activating the immune system.

Infectious disease

A disease acquired from another human, or sometimes from animals. When an infectious disease is acquired, it means the pathogen has entered the body and started to multiply causing damage to tissues in the body.

Inflammation

A process that occurs when the immune system identifies something foreign in the body. This can appear as redness, swelling and pain. Inflammation may occur at the injection site after a vaccine, because this is a normal and expected response that shows the vaccine is being effective.

Microorganisms

Very small living things, including bacteria, viruses, and parasites. Microorganisms that cause disease are called pathogens.

Pathogen

Any kind of infectious organism that causes disease.

Vaccine

The substance used for immunisation. Vaccination refers to the act of giving a vaccine to a person.

Virus

A tiny infectious agent that needs cells from other organisms to survive and multiply.

Immunisation is the most successful form of disease prevention available today and will continue to be an essential tool for controlling infections and their complications.¹ The science behind immunisation and vaccine development is well established after decades of research.^2–4 However, it can be challenging for many people to understand how immunisation works or find answers to questions and concerns about vaccination.

This guide aims to summarise the science of immunisation by answering five key questions:

1 What is immunisation?

The purpose of immunisation, achieved by using vaccines, is to prevent people from developing infectious diseases and to protect them against short- and longer-term complications. 

Find out more about immunisation

2 What is in a vaccine?

Vaccines generally contain two main types of active ingredients: antigens, which usually consist of parts of the pathogen and are designed to cause the immune system to produce a specific immune response; they may also contain adjuvants, which amplify the body’s immune response.

Find out more about what is in a vaccine

3 Who benefits from vaccines?

Individuals benefit from personal protection, and the wider community benefits from most vaccines because of herd immunity. The benefits of immunisation can sometimes include others, such as the babies of women vaccinated in pregnancy. Most importantly, vaccines prevent long-term serious complications that can arise from an infection.

Find out more about who benefits from vaccines

4 Are vaccines safe?

The vaccines currently in use in Australia provide benefits that greatly outweigh the risks of associated adverse events or side effects.

Safety research and testing is an essential part of vaccine development and manufacture. Before vaccines are made available to the public, clinical trials must confirm safety and how well the vaccine works. Safety monitoring continues after vaccines have been introduced into the community.

Find out more about vaccine safety

5 What does the future hold for vaccination?

Vaccine technology continues to develop, with an increasing number of vaccines against many infectious diseases now available. The future of vaccination includes developing new technologies to deliver vaccines and generating new vaccines for both infectious and non-infectious diseases like cancer. In some cases, the effectiveness of existing vaccines is being improved.

Find out more about the future of vaccination

The Australian Academy of Science strives to create a scientifically informed community that is guided by and enjoys the benefits of scientific endeavour. Through its distinguished Fellows and its National Committees for Science, it is able to draw deeply on expertise from across the Australian science community to report on important scientific issues.

The Australian Academy of Science first published this report in 2012 to support public understanding of how vaccination protects us from infectious diseases. Two groups of internationally recognised scientists were formed to answer the six big questions that are most often asked about immunisation and vaccination science. An expert working group drafted the questions and answers, and an oversight committee comprehensively reviewed the answers to ensure they were based on the current state of knowledge. This 2016 edition has been fine-tuned to improve clarity and to reflect scientific advances.

The Academy is grateful for the pro-bono contributions made to this report by the contributing experts. The Academy also thanks the Australian Government Department of Health for providing financial support to prepare and publish the original version of this report as well as this refreshed edition.

Societies face choices about future climate change

Managing the risks from future human-induced climate change will necessarily be based on some combination of four broad strategies:

• Emissions reduction: reducing climate change by reducing greenhouse gas emissions.
• Sequestration: removing carbon dioxide (CO₂) from the atmosphere into permanent geological, biological or oceanic reservoirs.
• Adaptation: responding to and coping with climate change as it occurs, in either a planned or unplanned way.
• Solar geoengineering: large-scale engineered modifications to limit the amount of sunlight reaching the earth, in an attempt to offset the effects of ongoing greenhouse gas emissions.

Each embodies a large suite of specific options, with associated risks, costs and benefits. The four strategies can affect each other: for example, doing nothing to reduce emissions would require increased expenditure to adapt to climate change, and increased chances of future resort to geoengineering.

Options for emissions reduction centre on carbon dioxide

CO₂ is the dominant contributor to human-induced climate change (Question 3). If the world adopts a target of keeping warming to less than 2°C above preindustrial temperatures, then future cumulative CO₂ emissions would need to be capped at around 30 years worth of current emissions (Question 4). Estimates of the amount of carbon in accessible fossil fuel reserves vary, but all agree that these reserves are at least several times larger than the carbon cap for a 2°C warming limit. Therefore, such a carbon cap, or even a significantly more lenient one, can only be met if a large fraction of available fossil fuel reserves remains unburned or if the CO₂ released is captured and permanently sequestered (see below).

Methane, nitrous oxide, halocarbon gases and black-carbon aerosols also have warming effects (Question 3), and reductions in their emissions would reduce the near-term warming rate. However, their combined contributions to warming over the longer term would be much less than that of CO₂, so these reductions alone could not meet a goal such as a 2°C warming limit.

There are many ways to reduce emissions of CO₂ and other warming agents, including shifting energy supply away from dependence on fossil fuels; energy efficiency in the domestic, industrial, service and transport sectors; reductions in overall demand through better system design; and efficient reductions in emissions of methane, nitrous oxide, halocarbon gases and black-carbon aerosols. Uptake of all of these options is happening now, and multiple studies have shown that they can be expanded effectively.

Other options are available but have significant collateral effects

In principle there are two interventions that could relax constraints on future emissions, but with significant uncertainties, risks, costs, and/or limitations. One would be to remove CO₂ from combustion exhaust streams or from the air, and sequester it underground, in the deep ocean, or in trees or the soil. The places used to store this carbon need to hold it for many centuries. Such carbon sequestration strategies face logistical, economic and technical challenges.

The other possible intervention would be to reduce Earth’s net absorption of sunlight, for example by generating a stratospheric aerosol layer or placing shields in space. While this could offset the surface warming caused by increasing greenhouse gases, it would do nothing to stop ocean acidification, would need to be maintained in perpetuity, and would carry multiple risks of adverse additional consequences on a global scale. Our current understanding of the climate system does not enable us to fully understand the implications of such actions.

Some climate change is inevitable and adaptation will be needed

Under any realistic future emissions scenario (Question 4), some additional global warming is inevitable and will require adaptation measures. Indeed, adaptation is needed now in response to climate change that has occurred already. The more CO₂ that is emitted in the next few decades, the stronger the adaptation measures that will be needed in future. There are limits to the adaptive capacities of both ecosystems and human societies, particularly in less developed regions. Thus, the decisions we make today on emissions will affect not only the future requirements for and costs of adaptation measures, but also their feasibility.

Decisions are informed by climate science, but fundamentally involve ethics and value judgements

As our society makes choices about managing the risks and opportunities associated with climate change, there is an important role for objective scientific information on the consequences of alternative pathways. Choices also hinge on ethical frameworks and value judgements about the wellbeing of people, economies and the environment. The role of climate science is to inform decisions by providing the best possible knowledge of climate outcomes and the consequences of alternative courses of action.

A number of factors prevent more accurate predictions of climate change, and many of these will persist

While advances continue to be made in our understanding of climate physics and the response of the climate system to increases in greenhouse gases, many uncertainties are likely to persist. The rate of future global warming depends on future emissions, feedback processes that dampen or reinforce disturbances to the climate system, and unpredictable natural influences on climate like volcanic eruptions. Uncertain processes that will affect how fast the world warms for a given emissions pathway are dominated by cloud formation, but also include water vapour and ice feedbacks, ocean circulation changes, and natural cycles of greenhouse gases. Although information from past climate changes largely corroborates model calculations, this is also uncertain due to inaccuracies in the data and potentially important factors about which we have incomplete information.

It is very difficult to tell in detail how climate change will affect individual locations, particularly with respect to rainfall. Even if a global change were broadly known, its regional expression would depend on detailed changes in wind patterns, ocean currents, plants, and soils.

The climate system can throw up surprises: abrupt climate transitions have occurred in Earth’s history, the timing and likelihood of which cannot generally be foreseen with confidence.

Despite these uncertainties, there is near-unanimous agreement among climate scientists that human-caused global warming is real

It is known that human activities since the industrial revolution have sharply increased greenhouse gas concentrations; these gases have a warming effect; warming has been observed; the calculated warming is comparable to the observed warming; and continued reliance on fossil fuels would lead to greater impacts in the future than if this were curtailed. This understanding represents the work of thousands of experts over more than a century, and is extremely unlikely to be altered by further discoveries.

Uncertainty works in both directions: future climate change could be greater or less than present-day best projections

Any action involves risk if its outcomes cannot be foreseen and the possibility of significant harm cannot be ruled out. Uncertainty about the climate system does not decrease risk associated with greenhouse gas emissions, because it works in both directions: climate change could prove to be less severe than current estimates, but could also prove to be worse.

Even if future changes from greenhouse gas emissions are at the low end of the expected range, a high-emissions pathway would still be enough to take the planet to temperatures it has not seen for many millions of years, well before humans evolved. In this situation, there can be no assurance that significant harm would not occur.

Science has an important role in identifying and resolving uncertainties, and informing public policy on climate change

All societies routinely make decisions to balance or minimise risk with only partial knowledge of how these risks will play out. This is true in defence, finance, the economy and many other areas. Societies have faced and made choices about asbestos, lead, CFCs, and tobacco. Although each case has unique aspects, all carried scientifically demonstrated but hard-to-quantify risks, and were contentious, in common with climate change.

Mechanisms have been put in place nationally and internationally to facilitate scientific input into decision making. In particular, the international Intergovernmental Panel on Climate Change (IPCC) has prepared thorough, ‘policy-neutral but policy-relevant’ assessments of the state of knowledge and uncertainties of the science since 1990, with the most recent assessment completed in 2014. Australian scientists have made a major contribution to the quality and integrity of these international IPCC assessments.

Climate changes have always affected societies and ecosystems

Climate change, whatever the cause, has profoundly affected human societies and the natural environment in the past. Throughout history there are examples of societal collapse associated with regional changes in climate, ranging from the decline of the Maya in Mexico (linked to drought) to the disappearance of the Viking community from Greenland in the fifteenth century (linked to decreasing temperatures). Some of these regional climate changes occurred rapidly, on timescales similar to current rates of global climate change.

Impacts from human-induced climate change are already occurring

The clearest present-day impacts of climate change in Australia and elsewhere are seen in the natural environment, and are associated with warming temperatures and increases in the number, duration and severity of heatwaves. These impacts include changes in the growth and distribution of plants, animals and insects; poleward shifts in the distribution of marine species; and increases in coral bleaching on the Great Barrier Reef and Western Australian reefs. Some of these changes can directly affect human activities; for example, through the effects of changing distributions of fish and other marine organisms on commercial and recreational fisheries, and the impacts of coral bleaching on tourism.

Some regional changes in Australian rainfall have been linked to human induced climate change. Southwest Western Australia has experienced a reduction in rainfall since the 1970s that has been attributed, at least in part, to enhanced greenhouse warming (Question 3). Societal adaptation to the resulting shortfalls in water supply is possible and already occurring (Box 7.1).

Box 7.1: Impacts of a drier climate: the case of southwest Western Australia

Declining rainfall and surface reservoir recharge since the mid-1970s in southwest Western Australia have been linked to changes in atmospheric circulation that are consistent with what would be expected in an atmosphere influenced by increasing greenhouse gas concentrations. The Water Corporation of Western Australia is addressing the diminishing surface water resource by setting out to deliver a ‘climate-independent’ supply of water for domestic consumption through two desalination plants. These now have the capacity to provide around half the piped water supply for the wider Perth region at a cost several times greater than that of surface water.

Current changes are expected to continue and intensify in the future

The impacts of future climate change and related sea-level rise will be experienced in many areas, from the natural environment to food security and from human health to infrastructure.

Ecosystems: Among Australia’s terrestrial ecosystems, some of the most vulnerable to climate change are (1) alpine systems as habitats shift to higher elevations and shrink in area; (2) tropical and subtropical rainforests due to warming temperatures (moderated or intensified by rainfall changes); (3) coastal wetlands affected by sea-level rise and saline intrusion; (4) inland ecosystems dependent on freshwater and groundwater that are affected by changed rainfall patterns; and (5) tropical savannahs affected by changes in the frequency and severity of bushfires.

Climate warming causes land and ocean life to migrate away from areas that have become too warm, and towards areas that previously were too cool. In many places, climate change is likely to lead to invasion by new species and extinctions of some existing species that will have nowhere to migrate, for example because they are located on mountain tops (Figure 7.1). Seemingly small changes, such as the loss of a key pollinating species, may potentially have large impacts.

Carbon dioxide affects ecosystems directly, both positively and negatively. On land it enhances growth in some trees and plants, an effect sometimes called ‘CO₂ fertilisation’. Absorption of CO₂ into the oceans causes ‘ocean acidification’, impeding shell formation by organisms such as corals and causing coral deterioration or death.

Bushfires: The number of extreme fire risk days has grown over the past four decades, particularly in southeast Australia and away from the coast (Figure 7.2). Future hotter and drier conditions, especially in southern Australia, are likely to cause further increases in the number of high fire-risk days and in the length of the fire season. CO₂ fertilisation may lead to increased foliage cover and hence increased fuel loads in warm arid environments such as parts of southern Australia. A study of southeast Australia has projected that the number of fire danger days rated at ‘very high’ and above could double by 2050, under high emission climate scenarios. Whether or not this leads to more, or worse, fires, and hence to changes in ecosystems, agriculture and human settlements, will depend on how this risk is managed.

Food security: In a non-drought year, around three-quarters of Australian crop and livestock production is exported. The range of adaptation strategies for primary producers to meet the challenge of climate change is large, including breed and seed selection, water conservation and changes in the timing of farm operations. Over the next few decades, some Australian agriculture may benefit from warmer conditions and from the fertilisation effect of increased CO₂ in the atmosphere. Looking further into the future, much depends on the effects of climate change on rainfall regimes in Australia’s farming regions. If rainfall increases, climate change may continue to be beneficial for some agriculture. However, for drier, hotter, higher variability climate change scenarios, there are limits to adaptation with anticipated declines in crop yield and livestock production.

Health: Heatwaves are among the highest-impact climate events in terms of human health in Australia. In very hot conditions, people can suffer from heat stress, especially vulnerable individuals such as the sick and elderly. During the heatwave of early 2009 in Victoria, there were 374 more deaths than average for the time of year (Figure 7.3). Warmer temperatures in future will lead to increased occurrences of heatwaves (Figure 5.2 left). Without further adaptation, extremely hot episodes are expected to have the greatest impact on mortality in the hotter north, while in cooler southern Australia there is likely to be an offsetting reduction in the number of cold-season deaths.

Warmer temperatures may lead to an increase in diseases spread via water and food such as gastroenteritis. Over the next few decades, Australia is expected to remain malaria-free. However, other vector-borne diseases such as dengue fever, Barmah Forest Virus and Ross River Virus may expand their range, depending on socioeconomic and lifestyle factors related to hygiene, travel frequency and destinations, in addition to climate scenarios. Extreme events also have psychological impacts. Drought is known to cause depression and stress amongst farmers and pastoralists, and this impact may increase over southern Australia as a result of climate change.

Infrastructure: Climate change can have impacts on infrastructure such as electricity and transport networks. Electricity demand rises sharply during heatwaves because of increased air conditioning. To avoid extensive blackouts there has been investment in generation and network capacity that is only used for a short time. In New South Wales, capacity needed for fewer than 40 hours a year (less than 1% of time) accounts for around 25% of retail electricity bills. In the 2009 heatwave in Melbourne, many rail services were cancelled because rails buckled and air conditioning failed. Coastal inundation and erosion due to sea-level rise, particularly when accompanied by extreme weather events, pose risks to infrastructure.

Around 30,000 km of roads across Australia are at risk from a 1.1 metre sea-level rise, with housing and infrastructure at risk valued at more than $226 billion.

In engineering terms, adapting to some of these risks is straightforward. Perth recently experienced a heatwave more intense than the Melbourne event, but no trains were cancelled on the city’s more modern rail network. However, the costs of adapting infrastructure can be high.

Climate change will interact with the effects of other stresses

The impacts of climate change often act to amplify other stresses. For example, many natural ecosystems are already subject to urban encroachment, fragmentation, deforestation, invasive species, introduced pathogens and pressure on water resources. Some societies suffer warfare and civil unrest, overpopulation, poverty and sinking land in high population river deltas. Multiple stresses do not simply add to each other in complex systems like these; rather, they cascade together in unexpected ways. Therefore, climate change impacts, interacting with other stresses, have the potential to shift some ecosystems and societies into new states with significant consequences for human wellbeing. For moderate levels of climate change, developed countries such as Australia are well placed to manage and adapt to such cascading impacts. However, developing nations, especially the least developed, face risks from projected impacts that may exceed capacities to adapt successfully. As climate change intensifies, especially under high-emission pathways (Question 4), adaptive capacities may be exceeded even in developed countries.

The effects of climate change elsewhere will impact Australia

Human society is now globally interconnected, dependent on intricate supply chains and a finite resource base. The global population now exceeds 7 billion people and is expected to increase to 9.6 billion by 2050; half of all fresh water and almost a quarter of global plant productivity is appropriated for human use; forecast yield gaps for major crops are increasing, especially in developing countries, and some yields may be reaching biophysical limits; 145 million people live within one metre elevation of sea level, with around 72% of these in Asia.

In this interconnected world, many risks to Australia from climate change, and potentially many opportunities, arise from impacts outside our national borders. For example: (1) sea-level rise and extreme events will threaten coastal zones, Pacific small island states, and large urban centres in Asian megadeltas; (2) global food production and trading patterns will change as present-day exporters see production fall, and as new exporters emerge; (3) climate change may exacerbate emerging humanitarian and security issues elsewhere in the world, leading to increased demands on Australia for aid, disaster relief and resettlement.

The further global climate is pushed beyond the envelope of relative stability that has characterised the last several thousand years, the greater becomes the risk of major impacts that will exceed the adaptive capacity of some countries or regions. Australia is a wealthy, healthy and educated society well placed to adapt to climate change and with the capacity to help address the impacts of changing climates elsewhere in the world.

In past warmer climates, sea level was higher than today

Sea level was between 5 metres and 10 metres above current levels during the last interglacial period (129,000 to 116,000 years ago) when global average surface temperatures were less than 2°C above their values just before the start of the industrial era in the 19th century. The estimated contributions from ocean thermal expansion and a then smaller Greenland Ice Sheet imply a contribution also from Antarctica to this higher sea level.

Globally, sea levels are currently rising

For two thousand years before the mid-19th century, the long-term global sea-level change was small, only a few centimetres per century. Since then, the rate of rise has increased substantially; from 1900 to 2012, sea level rose by a global average of about 19 centimetres. In the past 20 years, both satellite and coastal sea-level data indicate that the rate of rise has increased to about 3 centimetres per decade. A similarly high rate was experienced in the 1920 to 1950 period (Figure 6.1).

The two largest contributions to sea-level rise since 1900 were the expansion of ocean water as it warmed, and the addition of water to the ocean from loss of ice from glaciers. Since 1990, there have been further contributions from surface melting of the Greenland ice sheet, and the increased discharge of ice into the ocean from both the Greenland and Antarctic ice sheets. This increase in ice-sheet discharge is related to increases in ocean temperatures adjacent to and underneath the glacier tongues and floating ice shelves that fringe the coast of Greenland and Antarctica. The sum of storage of water in terrestrial reservoirs and the depletion of ground water have made a small contribution to sealevel rise during the 20th century.

Australian sea levels are rising

Around the Australian coastline, sea level rose relative to the land throughout the 20th century, with a faster rate (partly as a result of natural climate variability) since 1993. This follows several thousand years when there was a slow fall of Australian sea levels relative to the land at rates of a few centimetres per century. This was a result of ongoing changes to the ‘solid’ Earth following loss of the large surface loading from ice sheets of the last ice age.

Sea levels are projected to rise at a faster rate during the 21st century than during the 20th century

By 2100, it is projected that the oceans will rise by a global average of 28 to 61 centimetres relative to the average level over 1986–2005 if greenhouse gas emissions are low, and by 52 to 98 centimetres if emissions are high (Figure 6.1). The largest contributions are projected to be ocean thermal expansion and the loss of ice from glaciers, with the Greenland ice sheet contributing from surface melt and ice discharge into the ocean. For Antarctica, increased snowfall may partially offset an increase in discharge of ice into the ocean. Observations indicate that an increased discharge from Antarctica is occurring, particularly from sectors of the Antarctic ice sheet resting on land below sea level. Recent models successfully simulate increased flow in individual Antarctic glaciers and support the rates of ice sheet loss that were used to estimate global sea level rise of up to 98 cm by 2100. However, the relevant ice-sheet processes are poorly understood and an additional rise of several tens of centimetres by 2100 cannot be excluded.

Regional sea-level change can be different from the global average because of changes in ocean currents, changes in regional atmospheric pressure, the vertical movement of land, and changes in the Earth’s gravitational field as a result of changes in the distribution of water, particularly ice sheets, on the Earth. For Australia, 21st century sea-level rise is likely to be close to the global average rise.

In addition to climate-driven sea-level change, local factors can also be important and may dominate at some locations. These include tectonic land movements and subsidence resulting from the extraction of ground water or hydrocarbons, sediment loading and compaction. Changes in sediment supply can affect local erosion/accretion of the coastline.

Rising sea levels result in a greater coastal flood and erosion risk

Rising average sea levels mean that extreme sea levels of a particular height are exceeded more often during storm surges. For the east and west coasts of Australia, this happened three times more often in the second half compared to the first half of the 20th century. This effect will continue with more than a ten-fold increase in the frequency of extreme sea levels by 2100 at many locations and a much increased risk of coastal flooding and erosion, even for a low emissions pathway.

Sea levels will continue to rise for centuries

By 2300, it is projected that high greenhouse gas emissions could lead to a global sea-level rise of 1 metre to 3 metres or more. This may be an underestimate because it is difficult to accurately simulate the changes in the discharge from the Antarctic and Greenland ice sheets.

Sustained warming would lead to the near-complete loss of the Greenland ice sheet over a thousand years or more, contributing up to about 7 metres to global average sea-level rise. This would occur above a warming threshold estimated to be between about 1°C and 4°C of global average warming relative to pre-industrial temperatures. It is possible that a larger sea-level rise could result from a collapse of sectors of the Antarctic ice sheet resting on land below sea level. Current understanding is insufficient to assess the timing or magnitude of such a multi-century contribution from Antarctica, although there is increasing evidence that it may already have commenced.

Australia has a variable climate with many extremes

With its iconic reference to ‘droughts and flooding rains’, Dorothea Mackellar’s 1904 poem My Country highlights the large natural variations that occur in Australia’s climate, leading to extremes that can frequently cause substantial economic and environmental disruption. These variations have existed for many thousands of years, and indeed past floods and droughts in many regions have likely been larger than those recorded since the early 20th century. This high variability poses great challenges for recording and analysing changes in climate extremes not just in Australia, but the world over. Nevertheless, some changes in Australia’s climate extremes stand out from that background variability.

Human-induced climate change is superimposed on natural variability

In a warming climate, extremely cold days occur less often and very hot days occur more often (Figure 5.1). These changes have already been observed. For example, in recent decades, hot days and nights have become more frequent, more intense and longer lasting in tandem with decreases in cold days and nights for most regions of the globe. Since records began, the frequency, duration and intensity of heatwaves have increased over large parts of Australia, with trends accelerating since 1970.

Because a warmer atmosphere contains more moisture, rainfall extremes are also expected to become more frequent and intense as global average temperatures increase. This is already being observed globally: heavy rainfall events over most land areas have become more frequent and intense in recent decades, although these trends have varied notably between regions and seasons. In southern Australia, for example, the frequency of heavy rainfall has decreased in some seasons. While there is no clear trend in drought occurrence globally, indications are that droughts have increased in some regions (such as southwest Australia) and decreased in others (such as northwest Australia) since the middle of the 20th century.

For other extreme weather events such as tropical cyclones, there are not yet sufficient good quality observational data to make conclusive statements about past long-term trends. However, as the climate continues to warm, intensification of rainfall from tropical cyclones is expected.

Recent scientific advances now allow us to begin ascribing changes in the climate system to a set of underlying natural and human causes. For example, it is now possible to estimate the contribution of human-induced global warming to the probabilities of some kinds of extreme events. There is a discernible human influence in the observed increases in extremely hot days and heatwaves. While the record high temperatures of the 2012/2013 Australian summer could have occurred naturally, they were substantially more likely to occur because of human influences on climate. By contrast, the large natural variability of other extremes, such as rainfall or tropical cyclones, means that there is still much less confidence in how these are being affected by human influences.

Extremes are expected to change in the future

As the climate continues to warm in response to further greenhouse gas emissions, high temperature extremes will become hotter and cold extremes will become less cold. The rate of change of temperature extremes in Australia will depend on future emission levels: higher emissions will cause progressively more frequent high extreme temperatures (Figure 5.2 left). Climate model projections also suggest (though with considerable uncertainty) that in the next several decades, heavy rainfall events in Australia will tend to increase under a high emissions pathway (Figure 5.2 right). Across the globe, projections point broadly to an intensification of the wettest days and a reduction in the return time of the most extreme events (Figure 5.3), although there is much regional variation in these trends. For Australia, a warmer future will likely mean that extreme precipitation is more intense and more frequent, interspersed with longer dry spells, likewise with substantial regional variability.

In many continents, including Australia, a high temperature event expected once in 20 years at the end of the 21st century is likely to be over 4°C hotter than it is today (Figure 5.4). Furthermore, what we experience as a one-in-20-year temperature today would become an annual or one-in-two-year event by the end of the 21st century in many regions.

Future changes in other extreme weather events are less certain. Evidence suggests there will be fewer tropical cyclones, but that the strongest cyclones will produce heavier rainfall than they do currently.

With continued strong growth in CO₂ emissions, much more warming is expected

If society continues to rely on fossil fuels to the extent that it is currently doing, then carbon dioxide (CO₂) concentrations in the atmosphere are expected to double from pre-industrial values by about 2050, and triple by about 2100. This ‘high emissions’ pathway for CO₂, coupled with rises in the other greenhouse gases, would be expected to result in a globalaverage warming of around 4.5°C by 2100, but possibly as low as 3°C or as high as 6˚C. A ‘low emissions’ pathway, based on a rapid shift away from fossil fuel use over the next few decades, would see warming significantly reduced later this century and beyond (Figure 4.1).

During the next few decades and beyond, global warming is expected to cause further increases in atmospheric moisture content, more extreme heatwaves, fewer frosts, further decreases in the extent and thickness of sea ice, further melting of mountain glaciers and ice sheets, shifts in rainfall (increases in most tropical and high-latitude regions and decreases in many subtropical and mid-latitude regions), further ocean warming, and further rises in sea levels. The magnitude of expected change depends on future greenhouse gas emissions and climate feedbacks.

Future projections, based on climate models operated across a large number of research centres worldwide, broadly agree on the patterns of global-scale warming, with greater atmospheric warming over land than over the oceans, and greater warming at high northern latitudes than in the tropics and Southern Ocean (Figure 4.2 top). Future changes depend on the emissions pathway, and will be less if emissions are curtailed than under a high emissions scenario. At more localised regional scales the models can produce different results: for example, some models project substantial changes to phenomena such as El Niño or dramatic changes to vegetation, and regional projections of precipitation vary between models (Figure 4.2 bottom).

Australia can expect further warming and changes in water availability

Australian temperatures are expected to rise by approximately half a degree or more by 2030 relative to 1990, bringing more hot days and nights. Average sea level is expected to be about 15 cm higher by 2030 relative to 1990 and some models project tropical cyclones becoming less frequent but more severe in peak rainfall intensity as the world warms.

It is likely that future rainfall patterns across Australia will be different from today. However, compared with temperature trends, changes in rainfall patterns are harder to predict. Regional rainfall projections from different climate models are frequently different from one another (e.g. over Australia; Figure 4.2). Nevertheless, some future trends are projected by a majority of models, including decreases over southwest Western Australia coastal regions. Future rainfall trends across the Murray Darling basin remain uncertain.

Changes in rainfall greatly affect water availability because changes in rainfall are amplified in the resulting changes in runoff to rivers: the runoff in typical Australian catchments changes by 2 to 3% for each 1% change in rainfall.

Long-term climate change is effectively irreversible

The decisions we make on carbon emissions over coming decades will affect our climate for a long time to come, as emissions will profoundly impact the rate of future climate change, particularly after 2030 (Figure. 4.1). Even if emissions of greenhouse gases are reduced to near zero during this century, we will have to live with a warmer climate for centuries. For those parts of the climate system that respond slowly, such as the deep ocean, ice sheets and permafrost, change will continue for a long time. Many associated impacts—such as sea-level rise— and processes that exacerbate climate change—such as releases of methane and CO₂ from thawing permafrost soils—will continue long after emissions are stopped.

These characteristics of the climate system mean that the only way to stop human-induced climate change (without resorting to ‘geoengineering’—the deliberate, large-scale modification of climate) is to reduce net greenhouse gas emissions to near-zero levels. The longer this takes to achieve, and the more greenhouse gases that are emitted in the meantime, the larger the scale of future climate change.

To keep global warming below any specified threshold, there is a corresponding limit on cumulative carbon dioxide emissions

The amount of future global warming is closely related to cumulative CO₂ emissions (Figure 4.3). For example, to have a 50:50 chance of keeping global average temperatures to no more than 2°C above preindustrial levels, the total CO₂ emitted from human activities (accounting also for effects of other gases) would have to stay below a ‘carbon quota’ between 820 and 950 billion tonnes of carbon. So far, humanity has emitted well over half of this quota: between 1870 and 2013 cumulative emissions were 530 billion tonnes. The remaining quota is equivalent to around 30 years worth of current emissions. To stay within such a carbon quota, long-term global emissions reductions would have to average between 5.5% and 8% per year, accounting for time required to turn around present emissions growth.

Human activities have increased greenhouse gas concentrations in the atmosphere

Atmospheric concentrations of carbon dioxide (CO₂), methane and nitrous oxide began to rise around two hundred years ago, after changing little since the end of the last ice age thousands of years earlier. The concentration of CO₂ has increased from 280 parts per million (ppm) before 1800, to 396 ppm in 2013. This history of greenhouse gas concentrations has been established by a combination of modern measurements and analysis of ancient air bubbles in polar ice (Box 2.1).

Particularly important is CO₂. Enormous amounts of it are continually exchanged between the atmosphere, land and oceans, as land and marine plants grow, die and decay, and as carbon-rich waters circulate in the ocean. For several thousand years until around 200 years ago, this ‘carbon cycle’ was approximately in balance and steady. Since the 19th century, human-induced CO₂ emissions from fossil fuel combustion, cement manufacture and deforestation have disturbed the balance, adding CO₂ to the atmosphere faster than it can be taken up by the land biosphere and the oceans (Figures 3.1 and 3.2). On average over the last 50 years, about 25% of total CO₂ emissions were absorbed by the ocean making sea water more acidic and 30% was taken up on land, largely by increased plant growth stimulated by rising atmospheric CO₂, increased nutrient availability, and responses to warming and rainfall changes (though the mix of these mechanisms remains unclear). The other 45% of emissions accumulated in the atmosphere. These changes to the carbon cycle are known from measurements in the atmosphere, on land and in the ocean, and from modelling studies.

The dominant cause of the increasing concentration of CO₂ in the atmosphere is the burning of fossil fuels. Over the last two centuries, the growth of fossilfuel combustion has been closely coupled to global growth in energy use and economic activity. Fossilfuel emissions grew by 3.2% per year from 2000 to 2010 (Figure 3.3), a rapid growth that is dominated by growth in Asian emissions and has exceeded all but the highest recent long-range scenarios for future emissions.

Although fossil-fuel emissions of CO₂ have grown fairly steadily, the upward march of the CO₂ concentration in the atmosphere varies from year to year. This is caused mainly by the effects of weather variability on vegetation, and also by sporadic volcanic activity: major volcanic eruptions have a significant indirect influence on atmospheric CO₂ concentrations, causing temporary drawdown of CO₂ through the promotion of plant growth by the light-scattering and cooling effects of volcanic haze. By contrast, the direct contribution of volcanic emissions to atmospheric CO₂ is negligible, amounting to around 1% of current humaninduced emissions.

Most of the observed recent global warming results from human activities

Climatic warming or cooling arises from changes in the flows of energy through the climate system (Figure 1.1) that can originate from a number of possible driving factors. The main drivers that have acted over the last century are:

• increases in atmospheric CO₂ and other long-lived greenhouse gases (methane, nitrous oxide and halocarbons)
• increases in short-lived greenhouse gases (mainly ozone)
• changes to land cover (replacement of darker forests with paler croplands and grasslands)
• increases in aerosols (tiny particles in the atmosphere)
• solar fluctuations (changes in the brightness of the sun)
• volcanic eruptions.

Of these, solar fluctuations and volcanic eruptions are entirely natural, while the other four are predominantly caused by human influences. The human-induced drivers have been dominant over the past century (Figure 3.4). Changes in greenhouse gas concentrations, dominated by CO₂, caused a large warming contribution. Some of this has been offset by the net cooling effects of increased aerosol concentrations and their impact on clouds. Black carbon or soot has probably exerted a smaller, warming influence. The net effect of all aerosol types including soot remains hard to quantify accurately. Among the natural influences, the effect of changes in the brightness of the Sun has been very small (Box 3.1). Volcanic influences are highly intermittent, with major eruptions (such as Pinatubo in 1991) causing significant cooling for a year or two, but their average effects over the past century have been relatively small.

Box 3.1: Do changes in the Sun contribute to global warming?

In comparison with other influences, the effects of solar variations on present global warming are small. Indirect estimates suggest that changes in the brightness of the Sun have contributed only a few percent of the global warming since 1750. Direct measurements show a decreasing solar intensity over recent decades, opposite to what would be required to explain the observed warming. Solar activity has declined significantly over the last few years, and some estimates suggest that weak activity will continue for another few decades, in contrast with strong activity through the 20th century. Nevertheless, the possible effects on warming are modest compared with anthropogenic influences.

Using climate models, it is possible to separate the effects of the natural and human-induced influences on climate. Models can successfully reproduce the observed warming over the last 150 years when both natural and human influences are included, but not when natural influences act alone (Figure 3.5). This is both an important test of the climate models against observations and also a demonstration that recent observed global warming results largely from human rather than natural influences on climate.

It is also possible to distinguish the effects of different human and natural influences on climate by studying particular characteristics of their effects. For example, it was predicted more than a century ago that increases in CO₂ would trap more heat near the surface and also make the stratosphere colder. In recent years, satellite and other measurements have provided strong evidence that the upper atmosphere has cooled and the lower atmosphere has warmed significantly—the predicted consequence of extra greenhouse gases. This supports the inference that the observed nearsurface warming is due primarily to an enhanced greenhouse effect rather than, say, an increase in the brightness of the Sun.

Some recent changes in Australia’s climate are linked to rising greenhouse gases

Modelling studies indicate that rising greenhouse gases have made a clear contribution to the recent observed warming across Australia. Depletion of the ozone layer in the upper atmosphere over Antarctica and rising greenhouse gas concentrations are also likely to have contributed significantly to climate trends that have been observed in the Australian region over the past two decades. These include stronger westerly winds over the Southern Ocean, strengthening of the high-pressure ridge over southern Australia, and a related southward shift of weather systems. These trends are consistent with climate model projections, and are likely to be largely human-induced through a combination of increases in greenhouse gases and thinning of the ozone layer.

Past decadal trends in Australian rainfall (Question 2) cannot yet be clearly separated from natural climate variations, except in southwest Western Australia where a significant observed decline in rainfall has been attributed to human influences on the climate system.

There has very likely been net uptake of CO₂ by Australian vegetation, consistent with global uptake of CO₂ by vegetation on land (Figure 3.2). This has been accompanied by increases in the greenness of Australian vegetation, which is also consistent with global trends.