As heads of state gather in New York for tomorrow’s United Nations climate summit, a new report on the state of the world’s carbon budget tells them that greenhouse emissions hit a new record last year, and are still growing.
It shows that global emissions from burning fossil fuels and cement production reached a new record of 36 billion tonnes of CO2 in 2013, and are predicted to grow by a further 2.5% in 2014, bringing the total CO2 emissions from all sources to more than 40 billion tonnes. This is about 65% more fossil-fuel emission than in 1990, when international negotiations to reduce emissions to address climate change began.
Meanwhile, deforestation now accounts for just 8% of total emissions, a fraction that has been declining for several decades.
The growth of global emissions since 2009 has been slower than in the prior period of 2000-08. However, projections based on forecast growth in global gross domestic product (GDP) and continuance of improving trends in carbon intensity (emissions per unit of GDP) suggest a continuation of rapid emissions growth over the coming five years.
Global emissions continue to track the most carbon-intensive range among more than a thousand scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). If continued, this situation would lead to global average temperatures between 3.2C and 5.4C above pre-industrial levels by 2100.
There have been other striking changes in emissions profiles since climate negotiations began. In 1990, about two-thirds of CO2 emissions came from developed countries including the United States, Japan, Russia and the European Union (EU) nations. Today, only one-third of world emissions are from these countries; the rest come from the emerging economies and less-developed countries that account for 80% of the global population, suggesting a large potential further emissions growth.
Continuation of current trends over the next five years alone will lead to a new world order on greenhouse gas emissions, with China emitting as much as the United States, Europe and India together.
Country emission profiles
There are several ways to explore countries’ respective contributions to climate change. These include current emissions, per capita emissions, and cumulative emissions since the industrial revolution.
The largest emitters in 2013 were China, the United States, the 28 EU countries (considered as a single bloc), and India. Together, they account for 58% of global emissions and 80% of the emissions growth in 2013 (with the majority the growth coming from China, whereas the EU cut its emissions overall).
Here’s how the major emitters fared in 2013.
Emissions grew at 4.2%, the lowest level since the 2008 global financial crisis, because of weaker economic growth and improvements in the carbon intensity of the economy. Per capita emissions in China (7.2 tonnes of CO2 per person) overtook those in Europe (6.8 tonnes per person).
A large part of China’s high per capita emissions is due to industries that provide services and products to the developed world, not for China’s domestic use. China’s cumulative emissions are still only 11% of the total since pre-industrial times.
Emissions increased by 2.9% because of a rebound in coal consumption, reversing a declining trend in emissions since 2008. Emissions are projected to remain steady until 2019 in the absence of more stringent climate policies, with improvements in the energy and carbon intensity of the economy being offset by growth in GDP and population. The United States remains the biggest contributor of cumulative emissions with 26% of the total.
Emissions fell by 1.8% on the back of a weak economy, although reductions in some countries were offset by a return to coal led by Poland, Germany and Finland. However, the long-term decrease in EU emissions does not factor in the emissions linked to imported goods and services. When accounting for these “consumption” emissions, EU emissions have merely stabilised, rather than decreased.
Emissions grew by 5.1%, driven by robust economic growth and an increase in the carbon intensity of the economy. Per capita emissions were still well below the global average, at 1.9 tonnes of CO2 per person, although India’s total emissions are projected to overtake those in the EU by 2019 (albeit for a population nearly three times as large). Cumulative emissions account for only 3% of the total.
Emissions from fossil fuels declined in 2013, largely driven by a 5% decline of emissions in the electricity sector over the previous year (as shown by the Australian National Greenhouse Gas Accounts). Fossil fuel emissions per person remain high at 14.6 tonnes of CO2.
Is it too late to tame the climate?
Despite this apparently imminent event, economic models can still come up with scenarios in which global warming is kept within 2C by 2100, while both population and per capita wealth continue to grow. Are these models playing tricks on us?
Most models invoke two things that will be crucial to stabilising the climate at safer levels. The first is immediate global action to develop carbon markets, with prices rapidly growing to over US$100 per tonne of CO2.
The second is the deployment of “negative emissions” technologies during the second half of this century, which will be needed to mop up the overshoot of emissions between now and mid-century. This will involve removing CO2 from the atmosphere and storing it in safe places such as saline aquifers.
These technologies are largely unavailable at present. The most likely candidate is the production of bioenergy with carbon capture and storage, a combination of existing technologies with high costs and with environmental and socio-economic implications that are untested at the required scales.
There are no easy pathways to climate stabilization, and certainly no magic bullets. It is still open to us to choose whether we halt our CO2 emissions completely this century – as required for a safe, stable climate – or try instead to adapt to significantly greater impacts of climate change.
What we have no choice about is the fact that the longer emissions continue to grow at rates of 2% per year or more, the harder it will be to tame our climate.
Pep Canadell received support from the Australian Climate Change Science Program.
Michael Raupach has previously received funding from the Australian Climate Change Science Program, but does not do so now.
There is less than 1 chance in 100,000 that global average temperature over the past 60 years would have been as high without human-caused greenhouse gas emissions, our new research shows.
Published in the journal Climate Risk Management today, our research is the first to quantify the probability of historical changes in global temperatures and examines the links to greenhouse gas emissions using rigorous statistical techniques.
Our new CSIRO work provides an objective assessment linking global temperature increases to human activity, which points to a close to certain probability exceeding 99.999%.
Our work extends existing approaches undertaken internationally to detect climate change and attribute it to human or natural causes. The 2013 Intergovernmental Panel on Climate Change Fifth Assessment Report provided an expert consensus that:
It is extremely likely [defined as 95-100% certainty] that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic [human-caused] increase in greenhouse gas concentrations and other anthropogenic forcings together.
Decades of extraordinary temperatures
July 2014 was the 353rd consecutive month in which global land and ocean average surface temperature exceeded the 20th-century monthly average. The last time the global average surface temperature fell below that 20th-century monthly average was in February 1985, as reported by the US-based National Climate Data Center.
This means that anyone born after February 1985 has not lived a single month where the global temperature was below the long-term average for that month.
We developed a statistical model that related global temperature to various well-known drivers of temperature variation, including El Niño, solar radiation, volcanic aerosols and greenhouse gas concentrations. We tested it to make sure it worked on the historical record and then re-ran it with and without the human influence of greenhouse gas emissions.
Our analysis showed that the probability of getting the same run of warmer-than-average months without the human influence was less than 1 chance in 100,000.
We do not use physical models of Earth’s climate, but observational data and rigorous statistical analysis, which has the advantage that it provides independent validation of the results.
Detecting and measuring human influence
Our research team also explored the chance of relatively short periods of declining global temperature. We found that rather than being an indicator that global warming is not occurring, the observed number of cooling periods in the past 60 years strongly reinforces the case for human influence.
We identified periods of declining temperature by using a moving 10-year window (1950 to 1959, 1951 to 1960, 1952 to 1961, etc.) through the entire 60-year record. We identified 11 such short time periods where global temperatures declined.
Our analysis showed that in the absence of human-caused greenhouse gas emissions, there would have been more than twice as many periods of short-term cooling than are found in the observed data.
There was less than 1 chance in 100,000 of observing 11 or fewer such events without the effects of human greenhouse gas emissions.
Good risk management is all about identifying the most likely causes of a problem, and then acting to reduce those risks. Some of the projected impacts of climate change can be avoided, reduced or delayed by effective reduction in global net greenhouse gas emissions and by effective adaptation to the changing climate.
Ignoring the problem is no longer an option. If we are thinking about action to respond to climate change or doing nothing, with a probability exceeding 99.999% that the warming we are seeing is human-induced, we certainly shouldn’t be taking the chance of doing nothing.
The authors do not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article. They also have no relevant affiliations.
By Simon Torok
Here’s a simple backyard science experiment for you to try, which has global implications.
Grab a garden hose, turn it on, and then put your thumb over the end of it. The flow of water thins, while its power intensifies.
Okay, now multiply that by a few million and you have some idea of the impact of recent La Niña conditions on a major ocean current north of Australia.
The Indonesian Throughflow is a series of ocean currents linking the Pacific and Indian Oceans. It carries water from the Pacific to the Indian Ocean through the passages and straits of the Indonesian Archipelago.
Researchers – led by Janet Sprintall at Scripps Institution of Oceanography in the United States, and including Susan Wijffels from CSIRO in Hobart – have found that the flow of water in the Indonesian Throughflow has become more shallow and intense since the late 2000s due to La Niña conditions, just as the water flow thinned and intensified while you played with that garden hose.
The paper, The Indonesian seas and their role in the coupled ocean-climate system appears in today’s online publication of the journal Nature Geoscience.
The Indonesian Throughflow is the only place in the world where warm equatorial waters flow from one ocean to another; consequently, the throughflow is an important chokepoint in the flow of heat in the climate system.
The paper suggests that human-caused climate change could make this shallowing and intensification a more dominant feature of the Indonesian Throughflow, even under El Niño conditions.
Changes in how much warm water is carried by the Indonesian Throughflow will affect the sea surface temperature, and in turn the patterns of rainfall in our region.
So you may need to think a bit more about how you use that garden hose.
By Jaci Brown, CSIRO
We wait in anticipation of droughts and floods when El Niño and La Niña are forecast but what are these climatic events?
The simplest way to understand El Niño and La Niña is through the sloshing around of warm water in the ocean.
The top layer of the tropical Pacific Ocean (about the first 200 metres) is warm, with water temperatures between 20C and 30C. Underneath, the ocean is colder and far more static. Between these two water masses there is a sharp temperature change known as the thermocline.
Winds over the tropical Pacific, known as the trade winds, blow from east to west piling the warm top layer water against the east coast of Australia and Indonesia. Indeed, the sea level near Australia can be one metre higher than at South America.
Warm water and converging winds near Australia contribute to convection, and hence rainfall for eastern Australia.
In a La Niña event, the trade winds strengthen bringing more warm water to Australia and increasing our rainfall totals.
In an El Niño the trade winds weaken, so some of the warm water flows back toward the east towards the Americas. The relocating warm water takes some of the rainfall with it which is why on average Australia will have a dry year.
In the Americas El Niño means increased rainfall, but it reduces the abundance of marine life. Typically the water in the eastern Pacific is cool but high in nutrients that flow up from the deep ocean. The warm waters that return with El Niño smother this upwelling.
Have El Niño and La Niña always been around?
El Niño and La Niña are a natural climate cycle. Records of El Niño and La Niña go back millions of years with evidence found in ice cores, deep sea cores, coral and tree rings.
El Niño events were first recognised by Peruvian fisherman in the 19th century who noticed that warm water would sometimes arrive off the coast of South America around Christmas time.
Because of the timing they called this phenomenon El Niño, meaning “boy child”, after Jesus. La Niña, being the opposite, is the “girl child”.
Predicting El Niño and La Niña
Being able to predict an El Niño event is a multi-million, possibly billion dollar question.
Reliably predicting an impending drought would allow for primary industries to take drought protective action and Australia to prepare for increased risk of dry, hot conditions and associated bushfires.
Unfortunately each autumn we hit a “predictability barrier” which hinders our ability to predict if an El Niño might occur.
In autumn the Pacific Ocean can sit in a state ready for an El Niño to occur, but there is no guarantee it will kick it off that year, or even the next.
Nearly all El Niños are followed by a La Niña though, so we can have much more confidence in understanding the occurrence of these wet events.
A variety of events
Predictability would be even easier if all El Niños and La Niñas were the same, but of course they are not.
Not only are the events different in the way they manifest in the ocean, but they also differ in the way they affect rainfall over Australia – and it’s not straightforward.
The exceptionally strong El Niños of 1997 and 1982 have now been termed Super El Niños. In these events the trade winds weaken dramatically with the warm surface water heading right back over to South America.
Recently a new type of El Niño has been recognised and is becoming more frequent.
This new type of El Niño is often called an “El Niño Modoki” – Modoki being Japanese for “similar, but different”.
In these events the warm water that is usually piled up near Australia heads eastward but only makes it as far as the central Pacific. El Niño Modoki occurred in 2002, 2004 and 2009.
Australian rainfall is affected by all its surrounding oceans. El Niño in the Pacific is only one factor.
As a general rule though, the average rainfall in eastern and southern Australia will be lower in an El Niño year and higher in a La Niña. The regions that will experience these changes and the strength are harder to pinpoint.
El Niño and climate change
It is not yet clear how climate change will affect El Niño and La Niña. The events may get stronger, they may get weaker or they may change their behaviour in different ways.
Some research is suggesting that Super El Niños might become more frequent with climate change, while others are hypothesising that the recent increase in El Niño Modoki is due to climate change effects already having an impact.
Because climate change in general may decrease rainfall over southern Australia and increase potential evaporation (due to higher temperatures) then it would be reasonable to expect that the drought induced by El Niño events will be exacerbated by climate change.
Given that we are locked into at least a few degrees of warming over the coming century, it’s hard not to fear more drought and bushfires for Australia.
By Pep Canadell, Executive Director, Global Carbon Project
Record-breaking rains triggered so much new growth across Australia that the continent turned into a giant green carbon sink to rival tropical rainforests including the Amazon, our new research shows.
Published in the international journal Nature, our study found that vegetation worldwide soaked up 4.1 billion tons of carbon in 2011 – the equivalent of more than 40% of emissions from burning fossil fuels that year.
Unexpectedly, the largest carbon uptake occurred in the semi-arid landscapes of Australia, Southern Africa and South America.
It set a new record for a land-based carbon sink since high-resolution records began in 1958, in a remarkable example of ecosystems working to stabilise the Earth’s climate.
And that had a global impact. While atmospheric carbon dioxide still rose in 2011, it grew at a much lower rate – nearly 20% lower – than the average growth over the previous decade.
Almost 60% of the higher than normal carbon uptake that year, or 840 million tons, happened in Australia. That was due to a combination of factors, including geography and a run of very dry years, followed by record-breaking rains in 2010 and 2011.
Yet our research raises as many questions as it answers – in particular, about whether the Earth’s natural climate control mechanisms could prove even more volatile than previously thought.
The rain that made the world’s ocean fall
From October 2010 to March 2011, an extraordinary rainfall event occurred over most of Australia, which resulted in three-quarters of Queensland being declared a flood disaster zone – an area as big as France, Germany and Italy combined.
Averaged across Australia, the Bureau of Meteorology recorded rainfall of 703 millimetres for 2010 and 708 mm for 2011. That was well above the long-term average of 453 mm for the period of 1900 to 2009.
Excess rain reached most parts of the continent, in what proved to be the wettest two years combined since national climate records began in 1900.
Queensland was the worst affected area, with 35 people killed in floods that broke more than 100 river height records, and damaged 30,000 homes and businesses in cities and towns including Brisbane, Ipswich and Toowoomba. (You can see ABC News images of Brisbane before and after the floods here.)
The big rainfall event was part of a global phenomenon called the El Niño Southern Oscillation (ENSO), which reflects atmospheric pressure changes across the tropical Pacific Ocean, in its La Niña phase. It brought above-average rainfall not only to Australia but also to other parts of the world, particularly in southern Africa and northern South America.
The power of La Niña to evaporate water from the oceans was boosted by the ongoing high sea-surface temperatures that are part of a long-term trend of ocean warming. That trend has been shown to be associated with the release of greenhouse gases from the combustion of fossil fuels and deforestation.
This massive rain event was so significant that sensors on-board the twin satellites GRACE estimated a decrease in ocean water mass of 1.8 trillion tons. That remarkable finding was measured by changes in the Earth’s gravitational field, brought about by the transfer of water from the ocean to the atmosphere and land surface.
This made the ocean’s sea level fall by 5 millimetres from the beginning of 2010 to mid-2011, going against the average sea-level rise of 3mm a year over the previous 18 years associated with global warming.
Australia played a major role in this sea-level fall, for several reasons. It was partly due to vast amounts of rain that fell over Australia. The continent’s hydrological characteristics also played a role, with large impediments for rainfall to flow quickly back to the ocean, such as the large continental interior basins.
And Australia was a country in need of a big drink. The parched continent was emerging from a multi-year drought, particularly in the south-east region, meaning the land acted as a huge sponge, soaking up the heavy rainfall.
Seeing the Earth change colour from above
As a result of the unusually heavy rains, the Earth’s vegetation “greened” in 2011 in ways not measured over the previous 30 years, particularly in the Southern Hemisphere dryland ecosystems.
This global greening was detected by satellites, which observed increases in canopy foliage extent and vegetation water content, which both imply vegetation growth.
Combined, these measurements indicated that the world’s annual production of new plant matter significantly increased in 2011 when compared to the previous decade.
Regions in the Southern Hemisphere including Australia, southern Africa, and temperate South America contributed 80% of the change, especially their savannas and other semi-arid areas.
That winter, June to August 2011, Australia was the greenest that it has ever been seen in the satellite period (since 1982).
Our new study in Nature also shows how fire emissions – normally a big factor in reducing Australia’s capacity to store carbon – were suppressed by about 30%, contributing even further to the continent’s greening.
In addition to the unprecedented vegetation greening of Australia during 2010 and 2011, we also observe a greening trend over the continent since 1980s, particularly during the months of the Australian autumn (March, April, and May).
That has happened for a number of reasons, including increased continental rainfall over the past few decades; plants growing in an atmosphere with increasing carbon dioxide using water more efficiently; and changes in land management such as fire suppression, expansion of invasive species, and changes in livestock grazing that have led to more woodland.
The upsides of going green
Despite recurrent drought conditions in some regions, there is a current greening trend over Australia.
Overall, satellites show Australian landscapes are greener now than they have been over the past 30 years.
A greener Australia has a number of environmental and other benefits, including better protection for soils, increased soil-water holding capacity and soil fertility, and more plant feed to sustain larger animal populations.
However, more vegetation can lead to less water being available to replenish water tables and feed rivers, even though Australia loses more than 50% of all the rainfall to the atmosphere as soil evaporation, without contributing to vegetation growth.
This is in sharp contrast to temperate and tropical ecosystems, where a large part of the water is returned to the atmosphere via vegetation.
Fire, drought and rapid carbon release
However, we now need to consider whether this growing accumulation of carbon in semi-arid regions of the Southern Hemisphere could become a future climate liability through fire and drought.
Land and ocean carbon sinks absorb around half of the world’s emissions from burning fossil fuels each year, which helps to slow the rise of atmospheric carbon dioxide concentrations from human activities.
The Intergovernmental Panel on Climate Change’s Fifth Assessment Report found that we are likely to see an increase in climate variability that includes drier, more fire-prone conditions across large parts of the Southern Hemisphere’s semi-arid regions, including Australia.
That’s a vital trend to consider, because it could lead to a more vulnerable global carbon reservoir.
While we might see more carbon stored in new vegetation growth and soil when extra water is available in semi-arid regions, as happened in 2010-2011, the risk is that more fires and droughts would end up rapidly releasing that carbon back to the atmosphere.
It is likely that the large carbon uptake during 2011 was short-lived, as suggested by a rapid decline of the sink strength in 2012. Future research will be able to confirm if this was the case.
Arid and semi-arid regions currently occupy 40% of the world’s land area. More work is urgently needed to research the best ways to manage these areas, and whether we can increase their soil and vegetation carbon stores as part of our climate mitigation efforts.
While tropical forests like the Amazon remain vitally important as major carbon sinks, this new study and others indicate that semi-arid regions like Australia will also play a growing role in the Earth’s carbon cycle.
Increasingly, semi-arid regions are driving variability in how much carbon dioxide remains in the Earth’s atmosphere each year. And that has major implications for the long-term, including whether future climate change will slow down or accelerate further.
Pep Canadell receives funding from CSIRO and the Department of the Environment. This article is based on a new paper that he was a co-author of: Poulter, B, D Frank, P Ciais, R Myneni, N Andela, J Bi, G Broquet, JG Canadell, F Chevallier, YY Liu, SW Running, S Sitch and GR van der Werf. 2014. The contribution of semi-arid ecosystems to interannual global carbon cycle variability, Nature. Canadell’s contribution was supported by the Australian Climate Change Science Program.
The ’70s were all about disco, big hair, gold chains and flares… you can smell the hairspray just thinking about it.
But while the hairstyles were getting bigger and badder, scientists were busy making a discovery that would put them on a collision course with this emerging fashion.
The atmosphere’s ozone layer was being depleted – and CFCs (chlorofluorocarbons) were responsible. CFCs were one of the main chemicals in hairspray (as well as every other aerosol product) and were used in refrigerators and air conditioners.
When sunlight hits CFC molecules in the upper atmosphere, they break apart, producing a chlorine atom that in turn reacts with ozone molecules and breaks them apart – see this explanation from the Bureau of Meteorology. The ozone layer provides us Earthlings protection from the Sun’s harmful UV rays.
This discovery led to a landmark international agreement known as the Montreal Protocol of 1987, which saw most of the world’s countries sign on to phase out the use of CFCs. This has largely succeeded to date, with CFCs having been almost completely replaced by a related group of chemicals, hydrofluorocarbons (HFCs), which don’t deplete ozone (although they do have their own set of problems).
Or so we thought.
In research published in the journal Nature Geoscience this week, scientists revealed that they have detected four new ozone-depleting gases in our atmosphere. More than 74,000 tonnes of three new CFCs and one new hydrochlorofluorocarbon (HCFC) – an intermediate form of CFC – have been released into the atmosphere.
While this is a small amount when compared to the peak emissions of other CFCs in the ’80s, these emissions are contrary to what the Montreal Protocol set out to achieve – and so raise questions about where they are coming from.
The team, including our own Dr Paul Fraser, made the discovery by comparing today’s air samples with air trapped in polar firn (compacted snow), providing a natural archive of the atmosphere. They also looked at air samples collected between 1978 and 2012 at our Cape Grim air pollution station in northwest Tasmania.
The source of these new gases remains a mystery.
“We know they are coming from the northern hemisphere, but that is as good as we know at this stage,” Dr Fraser told ABC Science.
“It is good that we have found them quite early and that they haven’t accumulated to a significant degree in terms of ozone-depletion. Now we are hoping to find out where they are coming from so their sources can be switched off.”
By Leon Rotstayn, Senior Principal Research Scientist, Marine and Atmospheric Research
Climate scientists have established a convincing case for the link between increasing concentrations of greenhouse gases and observed warming of the Earth since the 19th century. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) stated, “Human influence on the climate system is clear.
“This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.” It also concludes that “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”
Aside from carbon dioxide, another human influence on climate comes from aerosols, which exert a cooling effect. Aerosols (that is, atmospheric particles, not propellants used in spray cans) have masked some of the warming that is caused by increasing greenhouse gases.
Without the masking effect of aerosols, global temperatures would have increased more than they have since the 19th century.
I recently led a study that examined the effects of declining aerosols in 21st century climate projections. We found that global warming is likely to accelerate in the next few decades, if the cooling influence of human-generated aerosols declines as predicted.
What are aerosols?
Aerosols are atmospheric particles, which have an overall cooling influence on climate by reflecting sunlight back into space. They also have an indirect effect by making clouds brighter; this further increases the reflection of sunlight back into space. Sources of human-generated aerosols include the use of fossil fuels and burning of vegetation.
Although human sources of aerosols are broadly similar to those of carbon dioxide, there is an important difference.
Emissions of carbon dioxide are intrinsically linked to the energy content of the fuel, so increasing energy use leads to increasing emissions of carbon dioxide. But aerosols are produced as a by-product of the combustion process, and in many cases there are technologies that can reduce emissions of aerosols (or gases that subsequently form aerosols in the atmosphere).
Because aerosols have harmful effects on human health and the environment, such technologies have been deployed in the industrialised world for some time. As long ago as the mid-1970s, emissions of sulfur dioxide from coal-fired power stations started to decline in Europe and North America, due to controls that were introduced to combat acid rain, which was destroying forests. These controls also had the effect of reducing sulfate, an aerosol that exerts cooling effects on climate.
More recently, authorities in China recognised the problems caused by aerosol pollution, and began to introduce emission controls, similar to those first seen in the industrialised world in the 1970s. Observations in China show that aerosol pollution peaked in 2006, and has started to decrease since then, despite continuing rapid economic growth. However, high levels of aerosol pollution are still causing serious concerns about health effects in China, suggesting that there is a strong need to further reduce emissions.
In many other developing countries, aerosol emissions are still increasing.
Over the last several years, climate modellers from many research centres carried out new climate projections, which provided input to the IPCC Fifth Assessment Report. These climate projections are driven by a range of scenarios (or “pathways”), which have different assumptions about changing levels of greenhouse gases.
A common feature of all the pathways is that aerosol emissions decline sharply during the 21st century. The projected decline is based on the assumption that once wealth per capita reaches a certain level in each country, there will be an increased focus on cleaner, healthier air.
In other words, it is assumed that during the 21st century the developing world will follow a path similar to the industrialised world, where aerosol emissions have declined in recent decades.
What happens to climate if aerosols decline?
Whereas increasing aerosols have masked global warming in the past, projected declines in aerosol emissions would unmask the warming effects of increasing greenhouse gases.
We are currently going through a transition. Until recently, aerosols have been acting like a “handbrake” on global warming. Over the next few decades, the decline of aerosols is expected to accelerate global warming, adding to the effects of increasing greenhouse gases.
Results from CSIRO climate modelling suggest that the extra warming effect from a decline in aerosols could be about 1 degree by the end of the century. But the size of this effect is very uncertain, so we compared the results from the CSIRO model with those from a range of international models.
We found that models with a stronger aerosol cooling effect in the 20th century tend to simulate greater warming in the 21st century. In other words, climate models with a stronger aerosol masking effect also have a stronger unmasking effect as aerosols decline.
Understanding aerosol effects is one of the biggest challenges for climate scientists. Aerosol processes are highly complex, and the magnitude of aerosol cooling effects in today’s climate is uncertain.
Every aerosol plume contains a mind-boggling soup of different chemical species; some of these (most notably black carbon) actually exert warming effects on climate, partly offsetting the cooling effects of other species such as sulfate. It is also unclear whether aerosols will really decline as rapidly as assumed in the projections.
Aerosols present an intriguing policy challenge. Concerns about toxic effects of aerosols on health and the environment provide strong reasons to reduce their emissions. But a uniform reduction in aerosol emissions is expected to accelerate global warming.
Based on this research, scientists have suggested that selectively reducing black carbon emissions is a possible option for mitigating global warming that will also have important health benefits.
Leon Rotstayn receives funding from the Australian Government Department of the Environment through the Australian Climate Change Science Programme.
More climate news on our Climate Response blog.