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Global Emissions Budgets Roundtable and Summary

As part of the Targets and Progress Review, the Authority is considering a number of broad questions on the latest climate changes science.

  1. What is the relationship between greenhouse gas emissions and temperature change over time?
  2. What are the current estimates for global emissions budgets likely to limit future temperature increases?
  3. What role do climate feedbacks and uncertainties play and how should they be considered in estimates of global emissions budgets?

To assist in these considerations, the Authority hosted an expert roundtable on 1 March 2013 on global emissions budgets. The purpose of the roundtable was to begin to develop the Authority's understanding of global emissions budgets and the latest climate science related to global emissions, projected temperature changes and uncertainties.

Eight prominent climate science experts attended the event and made presentations to the Authority on issues related to global emissions budgets.

A summary document has been prepared which presents the key issues discussed at the roundtable.

1. Overview

The Climate Change Authority (the Authority) is an independent body charged with providing advice to the Commonwealth Government on climate change policy. The Authority is currently conducting the Caps and Targets Review, which will address two broad topics:

  • Australia’s progress towards its medium and long term emissions reduction targets; and
  • Australia’s appropriate emissions reduction goals.

The Authority is required to report to the Commonwealth Government on the Caps and Targets Review (the Review) by 28 February 2014. Its report will include recommendations on a national emissions reduction target for 2020, and caps (that is, limits on emissions) for the first five years of trading under the carbon pricing mechanism.

Australia’s emissions reduction goals and actions are part of the broader context of global action, which in turn is guided by the common global objective to avoid dangerous climate change. This objective is broadly considered to mean that global warming (that is, the increase in global average temperature from pre-industrial levels) should be limited to no more than 2 degrees Celsius.

As part of this review, the Authority must therefore consider the current climate science and projections for future levels of climate change, and specifically, estimates of the global emissions budget.

To assist in these considerations, the Authority hosted an expert roundtable on 1 March 2013 on the broad topic of global emissions budgets. The purpose of the roundtable was to begin to develop the Authority’s understanding of global emissions budgets and the latest climate science related to global emissions, projected temperature changes and uncertainties.

Eight prominent climate science experts attended the event and made presentations to the Authority on issues related to global emissions budgets. The climate science experts have backgrounds in marine, atmospheric, earth and climate systems, complex systems science and mathematical physics, and plant physiology. Several of the scientists have contributed or led parts of the Intergovernmental Panel on Climate Change (IPCC) science assessment reports, and all have made contributions in peer reviewed journals and books.

The roundtable was designed as an introduction to key issues for non scientists, and was a valuable event for the Authority. This summary presents the primary issues discussed at the roundtable and, as a reflection of the roundtable itself, uses concepts and language for non-scientists, as far as possible. For further information, presentations made by the climate science experts can be found on the Authority’s website at www.climatechangeauthority.gov.au. Additional resources used to prepare this summary have also been identified in Appendix C.

The roundtable provided a starting point for the Authority’s considerations as it conducts the Caps and Targets Review, and this summary is an overview of some of the issues of interest to the Authority. Over the course of this Review, the Authority will continue to draw upon the latest climate science, further analyses, and input from consultation processes to review the global emissions budgets for carbon dioxide and other long-lived greenhouse gases in the context of determining Australia’s emissions reduction goals. The Authority expects to review and refine its understanding of the issues presented in this summary over the course of the Review, and welcomes any input to assist it in this task. The Authority proposes to release its draft report for the Caps and Targets Review in October 2013, and to provide opportunities for public comment on its draft findings and recommendations before finalising its views.

2. Background and context

Context and scope of the Climate Change Authority’s considerations

As an independent statutory body, the Authority’s work is guided by the Clean Energy Act 2011 (Cth) and the Climate Change Authority Act 2011 (Cth). One of the objects of the Clean Energy Act 2011 is to support the development of an effective global response to climate change, consistent with Australia’s national interest in ensuring that average global temperatures increase by not more than 2 degrees Celsius above pre-industrial levels.

In conducting any of its reviews, the Authority is also guided by a set of principles set out in the Climate Change Authority Act 2011. These include that measures to respond to climate change should:

  • be economically efficient, environmentally effective, equitable and in the public interest;
  • support the development of an effective global response to climate change, and be consistent with Australia’s foreign policy and trade objectives; and
  • take account of the impact on households, businesses, workers and communities.

Finally, the Clean Energy Act 2011 also sets out specific factors the Authority must consider in conducting the Caps and Targets Review. These include Australia’s international obligations and undertakings to reduce greenhouse gas emissions; global action to reduce emissions; and estimates of the global emissions budget.

With this context, the Authority has taken the global objective of limiting global average temperature increases by not more than 2 degrees as the anchor point for its consideration of the relevant climate science. The Authority was therefore interested in a number of broad questions for discussion at the roundtable:

  • What is the relationship between greenhouse gas emissions and temperature change over time?
  • What are the current estimates for global emissions budgets likely to limit future temperature increases?
  • What role do climate feedbacks and uncertainties play and how should they be considered in estimates of global emissions budgets?

This summary is divided into these three broad sections, and messages of most relevance to the Authority’s considerations are highlighted. Brief biographies of the climate science experts who attended the roundtable are included in Appendix A, and key terms are provided in Appendix B.

3. Greenhouse gases and temperature

KEY MESSAGES

  • Greenhouse gases trap and re-emit radiant heat within the atmosphere, which contributes to warming the earth through the so-called greenhouse effect.
  • Carbon dioxide is the principal greenhouse gas of concern: it is present in the highest concentrations of all greenhouse gases, it is very long-lived, and is influenced by human activities.
  • While the earth has experienced wide climate variations throughout its history, the rate of increase of greenhouse gas concentrations and temperature since the second half of the 20th century has been exceptional.
  • At present, the world is tracking towards a likely global average temperature increase of more than 2 degrees.

Greenhouse gases: the basics

Greenhouse gases make up a small proportion of the volume of gases in the atmosphere, and can have either natural or human sources. Greenhouse gases trap and re-emit radiant heat within the atmosphere, which contributes to warming the earth through the so-called greenhouse effect. Each greenhouse gas has a different capacity to trap radiant heat, resulting from both the different chemical compositions of each gas and their respective longevity in the atmosphere before they are removed by chemical or other processes.

The primary greenhouse gases in the atmosphere are water vapour, carbon dioxide, nitrous oxide, methane and ozone. There are also a number of entirely human-made greenhouse gases emitted into the atmosphere, such as halocarbons. Of all the greenhouse gases, carbon dioxide, methane, nitrous oxide, halocarbons, perfluorocarbons, sulfur hexafluoride and nitrogen trifluoride are both long-lived (relatively stable in the atmosphere for decades or more) and influenced by human activities, such as burning fossil-fuels to produce energy. Carbon dioxide is the principal greenhouse gas that affects the Earth’s radiative balance: it is present in the highest concentrations of all greenhouse gases and is very long-lived, with approximately 25 per cent of the increase in concentrations due to carbon dioxide emitted into the atmosphere over the last 50 years still present after 1000 years.

All greenhouse gases have a positive radiative forcing (warming) effect, as they increase the trapping of radiant energy emitted from the Earth within the atmosphere rather than allowing that energy to be released back into space. There are also agents that have a negative radiative forcing, or net cooling effect, as they increase the reflection of sunlight back into space. The most significant type of agent with a negative radiative forcing is aerosols, which generally have a short lifespan within the atmosphere, from several hours up to about 10 days. As policies to reduce air pollution, such as sulfate emissions from coal-fired electricity generation, are enacted, the cooling influence of aerosols is expected to diminish substantially over the 21st century.

Greenhouse gases and global temperature

The increasing concentration of greenhouse gases is problematic because of the positive forcing on climate, which is likely to have a significant effect on human life, the environment and other living things. The extent of the problem – how much warming has occurred, how much is too much, and how much can be attributed to human activities – has been the subject of significant attention and research.

There is very strong evidence for the link between higher atmospheric greenhouse gas concentrations and increased global temperatures. Relatively high greenhouse gas levels in the atmosphere have existed during periods of higher average temperatures, and periods of low greenhouse gas levels in the atmosphere have existed during periods with colder average temperatures and even ice ages. This has been established through samples of ice cores, deep sea cores and geological formations that have provided evidence of greenhouse gas levels, climate and environmental features at points or periods in time, dating from many thousands of years to millions of years ago.

While the earth has experienced wide climate variations throughout its history, the rate of increase of greenhouse gas concentrations and temperature since the second half of the 20th century has been exceptional relative to the past few thousand years at least. Figure 1 illustrates the rapid increase in carbon dioxide, methane and nitrous oxide concentrations over the past 1000 years as measured in the southern hemisphere.

Figure 1 Atmospheric concentrations of carbon dioxide, methane and nitrous oxide over the past 1000 years

Note: The figure shows 1000-year records of southern hemisphere background concentrations of carbon dioxide parts per million (ppm – orange), nitrous oxide parts per billion (ppb – blue) and methane (ppb – green) measured at Cape Grim Tasmania and in air extracted from Antarctic ice and near-surface levels of ice known as firn.

Source: CSIRO, 2012

Since the 1990s, the international policy community has commonly used a temperature increase above pre-industrial times of 2 degrees as the ‘guard rail’ level, beyond which the risk of dangerous interference with the climate is considered to be too high. The target of limiting average global warming to no more than 2 degrees is recognised in the objects of the Commonwealth Clean Energy Act 2011, and it is the shared vision of parties to the United Nations Framework Convention on Climate Change.

To have around a 50 per cent chance of limiting average global temperature increase to 2 degrees, it is widely regarded that greenhouse gas concentrations in the atmosphere would need to stabilise in the vicinity of 450 parts per million (ppm) of carbon dioxide equivalent. Above this, it becomes more likely than not that the global average temperature will increase by more than 2 degrees.

Figure 2 illustrates the relationship between concentrations of greenhouse gases in the atmosphere and the probability of limiting long term global average temperature increases to below a certain temperature.

Figure 2 Probability of staying below specific temperature increases above pre-industrial levels given carbon dioxide equivalent stabilisation levels

Source: Rogelj et al, 2012

As of 2011, the carbon dioxide concentration in the atmosphere was measured at 390 ppm. This is around 40 per cent higher than pre-industrial levels, and the highest level for at least the past 800 000 years based on paleoclimate estimates. Methane was 160 per cent above pre-industrial levels, and nitrous oxide 20 per cent. The combined radiative forcing due to all long-lived greenhouse gases was 473 ppm carbon dioxide equivalent. Taking into account the cooling effects of human-caused aerosols, the total radiative forcing due to human influences on the climate is equal to about 395 ppm carbon dioxide equivalent. At present, global carbon dioxide concentrations are rising by 2 ppm per year and carbon dioxide equivalent concentrations are rising by 3 ppm per year. The world is therefore tracking towards a more-likely-than-not probability of exceeding 2 degrees unless there are very rapid global reductions in greenhouse gas emissions.

Projections based on recent emissions trends suggest four degrees of warming

Climate models project atmospheric greenhouse gas concentrations and expected temperature ranges into the future, and are based on physics of the climate system and alternative emissions scenarios that involve different assumptions on economics, demographics, society and technology. The IPCC publishes scenarios and temperature projections in their Assessment Reports, which are considered to provide the most comprehensive and robust projections of the possible temperature ranges associated with different greenhouse gas concentrations.

When the actual global emissions between 2000 and 2007 are compared against the IPCC’s Fourth Assessment Report (2007) climate model scenarios, it is evident that the world’s emissions have been tracking near the high emissions scenarios (see Figure 3). Should the world’s emissions continue this trend, climate models predict that global average temperatures are likely to increase by 2.9–6.9 degrees above pre-industrial levels by 2100, with the best estimate being 4.5 degrees.

Figure 3 Actual carbon dioxide emissions from fossil fuels compared with Intergovernmental Panel on Climate Change Fourth Assessment Report Scenarios

Note: PgC/y=petagrams of carbon per year. A petagram is 1 billion tonnes, which is the same as a gigatonne.

Source: CSIRO, 2011; Figure 2.6

A 2012 report from the World Bank – Turn down the heat – found that even if emissions mitigation commitments and pledges made by nations at climate negotiations in Cancun in 2010 are met, there is approximately a 20 per cent chance the world will warm by more than an average of 4 degrees. If the mitigation commitments and pledges are not met, 4 degrees of warming could occur as early as the 2060s.

4. Global emissions budgets

KEY MESSAGES

  • Emissions budgets set an overall limit on global emissions, by estimating, within a probability range, the likely global average temperature increase that will result from a given level of greenhouse gas emissions over time.
  • For any given budget, there are a number of possible emissions trajectories.
  • To have a 75 per cent chance of limiting global average warming to 2 degrees above pre-industrial times, the world has a cumulative carbon dioxide emissions budget of around 1000 billion tonnes of carbon dioxide over the period 2000–2050.
  • More than 35 per cent of the allowable emissions under this budget have already been emitted.

The emissions budget approach

As discussed in the previous section, there is very strong evidence for the link between atmospheric greenhouse gas concentrations and temperature. Emissions budgets estimate, within a probability range, the likely global temperature increase that will result from a given level of cumulative emissions in the atmosphere. The emissions budget approach thus links cumulative emissions of greenhouse gases directly to temperature, without focusing on the intermediate steps displayed in Figure 4. The relationship can only be expressed as a probability, because there is significant uncertainty around how the climate will respond to a given amount of greenhouse gas emissions.

Figure 4 Architecture of connections between climate goals

The emissions budget approach is a useful concept because while budgets identify the overall limit on global emissions, they do not assume particular timing of peak emissions or the rate at which emissions are reduced, so long as the overall quota is not breached. This means that policy makers can consider different rates of emissions reductions based on economic, social and technology considerations.

However, because the emissions budget is ultimately fixed, delays in reducing emissions must be compensated with more rapid emissions reductions in future years.

There are three different kinds of emissions budgets. The first and most researched type of budget (carbon dioxide emissions in a carbon dioxide-only world) focuses on the quota of cumulative carbon dioxide emissions allowed when you model only the radiative forcing of carbon dioxide (e.g. ignoring the effect of other forcing agents, both warming and cooling). A more representative budget is one that sets a quota of cumulative carbon dioxide emissions where the radiative forcing of all atmospheric constituents is taken into account (carbon dioxide emissions in an all-forcing world). The third type of budget includes the cumulative emissions of all greenhouse gases influenced by human activities, measured in carbon dioxide equivalent emissions, in an all-forcing world (greenhouse gas emissions in an all-forcing world). This type of budget is most closely aligned with the Kyoto Protocol, but is scientifically less robust.

Each type of budget has its own strengths and weaknesses: a budget that considers only the radiative forcing of carbon dioxide will likely understate the expected change in climate, but including more forcing agents in a budget introduces more uncertainties regarding the climate response to other forcing agents. In general, because carbon dioxide is the longest-lived greenhouse gas, budgets considering only carbon dioxide emissions can give a good indication of the extent of likely long term temperature rise, and have advantages in the simplicity of their approach.

Global budgets consistent with 2 degrees

In a 2009 study by Meinshausen et al, a 1000 billion tonne budget of carbon dioxide-only emissions in an all-forcing world for the period 2000–2050 was estimated to give a 75 per cent chance of limiting temperature increases to no more than 2 degrees in the 21st century (assuming that the world economy is completely decarbonised from 2050).

Between 2000 and 2011, the world emitted 356 billion tonnes of carbon dioxide, more than 35 per cent of the total emissions allowed under this budget. That means that from 2012, the world had a remaining budget of around 640 billion tonnes of carbon dioxide which could be emitted to 2050. Emissions are currently growing at approximately 2.5 per cent per annum. At current rates, the entire quota of emissions allowable under this scenario would be used up by 2028.

Accepting a higher or lower likelihood of exceeding a 2 degree temperature rise gives different budget figures. For example, accepting a 50 per cent probability of limiting warming to 2 degrees increases the allowable budget to 1437 billion tonnes of carbon dioxide. Requiring an 80 per cent probability of limiting warming to 2 degrees reduces the allowable budget to approximately 890 billion tonnes of carbon dioxide.

As discussed in the previous section, different types of budgets will also give different figures, depending on what emissions are being counted and what type of world is modelled. For example, the same 2009 Meinshausen study also estimates the allowable budget for all Kyoto Protocol listed greenhouse gases in an all-forcing world for the same period. For an approximately 75 per cent chance of limiting temperature increases to no more than 2 degrees, the budget for 2000–2050 is estimated to be 1 500 billion tonnes of carbon dioxide equivalent emissions. For a 50 per cent chance, the allowable budget increases to 2 000 billion tonnes of carbon dioxide equivalent emissions.

Figure 5 illustrates the 1 000 billion tonne carbon dioxide budget (blue line in the top panel), compared with carbon dioxide emissions under a no climate policy scenario (red line). These emissions projections correspond to the probabilities of limiting global warming to different levels by 2100 in the bottom panel. With higher emissions budgets, the probability of staying below 2 degrees by the end of the 21st century decreases.

Figure 5 Probability of staying below 2 degrees with a 1 000 billion tonne carbon dioxide budget to 2050, versus business as usual

GtC02 /yr= gigatonnes of carbon dioxide per year. A gigatonne is 1 billion tonnes, which is the same as a petagram

Source: Australian Academy of Science, 2010; Figure 5.1

5. Feedbacks and uncertainties

KEY MESSAGES

  • There are numerous sources of uncertainty in any estimates of future temperature change and associated climate impacts, resulting in wide probability ranges.
  • The ability of the climate system to continue to absorb carbon dioxide through land and ocean sinks at a pace consistent with emissions is a key source of uncertainty, as is the possible role of positive feedbacks (such as melting permafrost) as temperatures increase.
  • The collective policy implication of these uncertainties is that a precautionary allowance may need to be made when considering an emissions budget for a given temperature limit: this would invariably lower the amount of allowable emissions from human activities.

One of the most important elements of the Authority’s considerations of global emissions budgets is appreciating that there are inherent uncertainties in any estimates of future temperature change. A number of these are common to many scientific and other endeavours, such as incomplete data, imperfect modelling and the difficulties in attempting to predict future states of the world. One uncertainty that is of particular interest when considering global emissions budgets is the response of the overall climate system to increasing concentrations of greenhouse gases. The climate system has shown an enormous capacity to absorb the large amount of carbon dioxide emissions resulting from human activities. To date, around 55 per cent of carbon dioxide emissions from fossil fuel combustion have been taken up by the land and oceans combined, with the remaining 45 per cent remaining in the atmosphere (Figure 6). A key question is whether these carbon sinks will continue to keep pace with emissions, as they have done in the past.

Figure 6 Sources and sinks of carbon dioxide

PgC y-1 = petagram of carbon per year. A petagram is 1 billion tonnes, which is the same as a gigatonne.

Source: CSIRO 2011, Figure 2.5

The fraction of the annual human emissions of carbon dioxide that has been taken up by the land has been relatively stable since records began at 28–30 per cent (with considerable variations year on year). The oceans have shown a decreasing trend, with the fraction of emitted carbon dioxide being taken up declining from around 32 per cent in 1960 to around 26 per cent in 2006. This decline is likely to be the result of a combination of factors, including that carbon dioxide is less soluble in warmer water and that global warming might be changing the circulation of the ocean such that there is less net transport of the surface well-mixed waters to the stable deep water. Consequently a larger proportion of carbon dioxide emissions remains in the atmosphere, which is where the greenhouse effect occurs.

Land sinks, such as soils and forests, have continued to take up atmospheric carbon dioxide as emissions have increased. This is probably partly due to higher levels of carbon dioxide in the air stimulating the rate of plant biomass formation by photosynthesis. Over time this natural land sink is expected to decline for plant physiological and ecological reasons but the time scale for that decline is unknown. A potential human-driven feedback in the climate system is through the idea that humans can foster uptake of atmospheric carbon dioxide into vegetation biomass and soil organic matter even faster than is happening naturally. Whether such human-induced additional land-based carbon sinks can play a significant role in removing even more atmospheric carbon dioxide, via market-based incentives, requires consideration of land, water and nutrient availability and costs.

In addition to possible changes in the current ability of the climate system to absorb carbon dioxide through land and ocean sinks, there are a number of possible positive feedbacks, where increased warming may cause an effect which will then itself result in additional warming. The most well-known of these possible positive feedbacks is the potential release of large volumes of carbon dioxide and methane from warming permafrost in the northern hemisphere (and also tropical peatland), which could become a major source of additional greenhouse gas emissions.

The policy implication of these uncertainties is that a precautionary allowance may need to be made for additional feedbacks when deciding on what cumulative emissions budget should be chosen for an acceptable level of global warming. This assessment would invariably lower the amount of allowable emissions from human activities.

Appendix A Climate science experts – short biographies

Dr. Josep Canadell

Dr. Josep Canadell is the executive director of the Global Carbon Project and research scientist in CSIRO Marine and Atmospheric Research, based in Canberra, Australia. His work focuses on global and regional interdisciplinary analyses and syntheses on a variety of topics of the carbon cycle and its disturbance by human activities. This includes the global carbon budget (carbon dioxide and methane), regional carbon budgets and attribution to flux components, the size and vulnerability of earth’s carbon pool (carbon–climate feedbacks), and the role of land-based mitigation activities in climate stabilisation. He is leading author of the Fifth Assessment Report of the IPCC.

Professor David Karoly

Professor David Karoly is Professor of Climate Science in the University of Melbourne’s School of Earth Sciences, and is a member of the Climate Change Authority. His research expertise is in climate variability and climate change, including greenhouse climate change, stratospheric ozone depletion and interannual climate variations due to El Niño-Southern Oscillation. Professor Karoly was Chair of the Premier of Victoria’s Climate Change Reference Group during 2008–09 and was invited to join the Commonwealth Government’s High Level Coordinating Group on Climate Change Science at the end of 2009. Since 2011 he has been a member of the Science Advisory Panel of the Australian Climate Commission, as well as the Joint Scientific Committee of the WMO/ICSU World Climate Research Programme. Professor Karoly is also a member of the Wentworth Group of Concerned Scientists and the Australian Academy of Sciences’ National Committee on Earth System Science.

Professor Michael Raupach

Professor Michael Raupach works on Earth System science, carbon-climate–human interactions, land–air interactions, and fluid mechanics. He is based at CSIRO Marine and Atmospheric Research. He is a Fellow of the Australian Academy of Science, the Australian Academy of Technological Sciences and Engineering, and the American Geophysical Union. From 2000 to 2008 he was an inaugural co-chair of the Global Carbon Project of the Earth System Science Partnership; in 2010 he chaired the Expert Working Group reporting to the Prime Minister’s Science, Engineering and Innovation Council (PMSEIC) on ‘Challenges at Energy-Water-Carbon Intersections’; and in 2010–2013 he led the Australian Academy of Science project ‘Negotiating our Future: Living Scenarios for Australia to 2050’.

Dr. Ian Enting

Dr. Ian Enting trained in mathematical physics at Monash University, followed by several postdoctoral appointments. In 1980, he joined CSIRO and worked on carbon cycle modelling. This involved both interpretive modelling (understanding the current behaviour of the carbon cycle) and predictive modelling (assessing the consequences of future choices for emissions patterns). All these studies were notable for their emphasis on systematic study of uncertainties. He was one of the lead authors for the carbon dioxide and the carbon cycle chapter of the report on Radiative Forcing of Climate for the IPCC.

Dr. Enting was extensively involved in developing the CSIRO Emerging Science Initiative in Complex Systems Science. In the area of interpretive modelling of the carbon cycle, Dr. Enting pioneered techniques for deducing net sources and sinks of carbon dioxide, giving observations of concentrations. His book Inverse problems in atmospheric constituent transport was published by Cambridge University Press in 2002.

Dr. Enting joined the ARC Center of Excellence for Mathematics and Statistics of Complex Systems (MASCOS) as a Professorial Research Fellow in May 2004, based at the University of Melbourne. His book Twisted: The distorted mathematics of greenhouse denial was published in 2007. He retired late in 2012, and continues as an honorary fellow at the University of Melbourne.

Dr. Malte Meinshausen

Dr. Malte Meinshausen is Senior Researcher at the Potsdam Institute for Climate Impact Research, Germany, and Honorary Senior Research Fellow at the School of Earth Sciences, The University of Melbourne. He holds a PhD on climate science and a Diploma in Environmental Sciences from the Swiss Federal Institute of Technology, as well as an MSc in Environmental Change and Management from the University of Oxford, UK. Before joining the Potsdam Institute for Climate Impact Research (PIK) in 2006, he was a postdoctoral fellow at the National Center for Atmospheric Research in Boulder, Colorado. He has been contributing author to various chapters in the Fourth Assessment Report of the IPCC. Until May 2011, he was leading the PRIMAP (‘Potsdam Real-time Integrated Model for Probabilistic Assessment of Emission Path’) research group at PIK before relocating to Melbourne. Since 2005, he is a scientific advisor to the German Environmental Ministry related to international climate change negotiations under the UNFCCC.

Professor Will Steffen

Professor Will Steffen is a climate and global change researcher at the Australian National University, Canberra. He served on the Multi-Party Climate Change Committee (MPCCC) in 2010–2011, and is a Climate Commissioner. From 1998 to mid-2004, Steffen served as Executive Director of the International Geosphere-Biosphere Programme, based in Stockholm, Sweden, and is currently a guest researcher at the Stockholm Resilience Centre. His research interests span a broad range within the fields of climate and Earth System science, with an emphasis on incorporation of human processes in Earth System modelling and analysis; and on sustainability and climate change.

Dr. Roger Gifford

Dr. Roger Gifford is a retired Chief Research Scientist, and now an Honorary Fellow, with CSIRO Plant Industry in Canberra. Trained as a plant physiologist and agronomist, he dedicated a large part of his 40-year research career to the impact of the increasing atmospheric carbon dioxide concentration, directly and indirectly via global warming, on plant processes and production, and the role of vegetation in the changing global carbon cycle.

Over the last several years, as Chairman of the National Committee or Earth System Science, he has led the process of developing a national strategic plan for Earth System Science research (‘To Live Within Earth’s Limits: An Australian Plan to Develop a Science of the Whole Earth System’). Deriving from a recommendation of that report, he has convened the first two Australian Earth System Outlook Conferences in 2010 and 2012.

Dr. Helen Cleugh

Dr. Helen Cleugh is an atmospheric scientist in CSIRO’s Marine and Atmospheric Research Division, where she is Deputy Chief Research (Climate and Atmosphere) and Deputy Director of the Centre for Australian Weather and Climate Research (CAWCR). She is based at CSIRO’s Black Mountain site in Canberra.

Dr. Cleugh obtained her PhD at the University of British Columbia, in Vancouver, Canada and worked for 7 years as a lecturer at Macquarie University before joining CSIRO in 1994. Her past achievements include leading the Australian National Windbreaks Program and OASIS – especially the coupled modelling of surface energy balance and convective boundary layer growth. In 2002 she was an Erskine Fellow at the University of Canterbury, New Zealand.

Dr. Cleugh has published over 60 refereed journal papers, books and book chapters. She has research expertise in the areas of micrometeorology and convective boundary layer dynamics; land–air fluxes of energy, water and carbon; and their effects on microclimates and hydrology of urban and non-urban ecosystems; assessments of evaporation and net ecosystem exchange of carbon dioxide in forested, cropping, vineyard, grassland, savanna and urban landscapes: directly using in situ eddy covariance technology and indirectly using MODIS remote sensing; urban water use, microclimate and energy use through measurements and modelling of urban energy, water balances and microclimates; and effects of windbreaks on airflow, surface energy balances, crop microclimates and productivity.

Appendix B Key terms

  • Aerosols

Aerosols are small solid particles or liquid droplets in the atmosphere and can have natural or human-made sources. Natural aerosols include dust and products of naturally occurring reactions, while those produced through human activities include smoke particles from burning fossil fuels, sulphate and fossil fuel carbon (organic and black carbon).

Aerosols reflect radiation back into space, either directly or indirectly (for example, by affecting the formation of clouds), and thus have a cooling effect.

Aerosols are generally only present in the atmosphere for a short period of time (between one and ten days), after which they are rained out or otherwise dissipate from the atmosphere.

  • Equivalent carbon dioxide concentration

The concentration of carbon dioxide that would cause the same amount of radiative forcing as a given mixture of carbon dioxide and other greenhouse gases.

  • Equivalent carbon dioxide emission

The amount of carbon dioxide emissions that would cause the same radiative forcing, over a given time period, as an amount of other greenhouse gases. The equivalent carbon dioxide emissions is obtained by multiplying the emissions of the other greenhouse gas by its Global Warming Potential for the given time period. Equivalent carbon dioxide emissions is a useful way to compare emissions of different greenhouse gases, but the corresponding climate change responses cannot necessarily be assumed to be equivalent.

  • Feedbacks

An interaction mechanism between processes in the climate system, when the result of an initial process triggers changes in a second process that subsequently influences the first process. A positive feedback intensifies the original process, and a negative feedback reduces it. An example of a positive feedback is the warming of the climate melting permafrost, which releases methane into the atmosphere, which reinforces the initial warming by contributing to the greenhouse effect.

  • Global Warming Potential

The strength of the radiative forcing from each long-lived greenhouse gas can be expressed in Global Warming Potential (GWP). GWPs equate the cumulative radiative forcing of each long-lived greenhouse gas to that of carbon dioxide, and are defined over a fixed time horizon (usually 100 years). GWPs are not used to measure the effect of short-lived greenhouse gases, such as water vapour.

  • Greenhouse gases

Greenhouse gases absorb and re-emit energy emitted from the Earth, the atmosphere itself and by clouds. The long-lived greenhouse gases that drive global temperature increases through prolonged radiative forcing in the atmosphere include carbon dioxide, methane, nitrous oxide, halocarbons, sulfur hexafluoride and nitrogen trifluoride.

  • Radiative forcing

Radiative forcing is the difference between the incoming radiant energy received by the Earth (for example, as heat and light energy received from the sun), and the radiant energy released from the Earth back into space.

All greenhouse gases have a positive radiative forcing effect, as they trap radiant energy radiated by the Earth within the atmosphere, rather than allowing that energy to be released back into space. This radiant energy has a warming effect on the Earth’s atmosphere and oceans. Each greenhouse gas has a different radiative forcing effect. There are also agents that have a negative radiative forcing, or net cooling, effect, as they allow radiant energy to be reflected into space. The most significant type of agent with a negative radiative forcing is aerosols.

Radiative forcing is a comparative point-in-time measure of the Earth’s energy balance compared with pre-industrial times. The year 1750, when atmospheric carbon dioxide levels were at 280 ppm, is used as the baseline and has a radiative forcing level of zero. Radiative forcing measurements therefore capture the effect of human activities as a result of industrialisation and land-use change. As emissions increase, the level of radiative forcing also increases (as radiative forcing is linked to the concentration of greenhouse gases in the atmosphere).

  • Sinks

Sinks refer to the way in which carbon is stored. Carbon is stored in the land, atmosphere and oceans. Land sinks refer to the take up of carbon dioxide through photosynthesis and other chemical processes that convert carbon dioxide to a different type of matter, such as plant sugars and oxygen. Atmospheric sinks refer to the retention of greenhouse gases in the atmosphere, where they have radiative forcing properties. Ocean sinks dissolve carbon dioxide from a gaseous state in the atmosphere to one in which the carbon dioxide is suspended in sea water.

Appendix C Bibliography

  1. Australian Academy of Science 2010, The science of climate change: Questions and answers, Australian Academy of Science, Canberra.
  2. CSIRO 2011, Climate change: Science and solutions for Australia, CSIRO Publishing, Collingwood.
  3. CSIRO 2012, State of the climate 2012, http://www.csiro.au/en/Outcomes/Climate/Understanding/State-of-the-Climate-2012/Greenhouse-Gases.aspx.
  4. Emission Database for Global Atmospheric Research, GHG (CO2, CH4, N2O, F-gases) emission time series 1990–2010 per region/country, http://edgar.jrc.ec.europa.eu/overview.php.
  5. German Advisory Council on Global Change 2009, Solving the climate dilemma: The budget approach, Special Report 2009, Berlin.
  6. Global Carbon Project, Full global carbon budget, 2012 Budget Version 1.5, http://cdiac.ornl.gov/trends/emis/meth_reg.html.
  7. Intergovernmental Panel on Climate Change 2007, Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge.
  8. Meinshausen, M et al. 2009, ‘Greenhouse-gas emission targets for limiting global warming to 2 °C’, Nature, vol. 458, 30 April 2009.
  9. Raupach, M, Harman, I and Canadell, J 2011, Global climate goals for temperature, concentrations, emissions and cumulative emissions, CAWCR technical report No. 042, Centre for Australian Weather and Climate Research, Melbourne.
  10. Rogelj, J, Meinshausen, M and Knutti, R 2012, ‘Global warming under old and new scenarios using IPCC climate sensitivity range estimates’, Nature Climate Change, vol. 2, April 2012.
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