This is an extract from The Science of Climate Change — Questions and Answers, published by the Australian Academy of Science and distributed to members of parliament, every local government authority in Australia and every Australian high school, in August 2010. Crikey will be running a series of extracts, including canvassing common myths.
Climate models and studies of past climates indicate that global warming and associated changes will continue if greenhouse gas levels keep rising as they are now
Basic physical principles tell us that rising levels of greenhouse gases will warm the Earth’s surface. To answer more complex questions, computer simulations, or models,
of the Earth’s climate are used. These models incorporate the many factors that affect our climate, using mathematical equations based on fundamental laws of nature, together with approximations of some physical processes that cannot be represented exactly (see Box 9).
Models simulate reasonably well the broad features of the present climate and the 20th century warming. This, however, does not guarantee accurate predictions into the future; changes could be more rapid or more gradual than projected. Overall, there is good agreement between models and observations at global and continental scales, but simulations are less reliable at the local scale.
Some properties of climate are better captured by models than others; for example, temperature is generally more accurately simulated than rainfall.
Independent of climate models, another important way to estimate the implications of greenhouse gas increases is to examine how climate has responded to such increases in the
past, both over geological time and in recent centuries.
While these two approaches – modelling and studying the past – rely on markedly different methodologies, they both yield broadly similar indications of where global climate is
headed. For example, both methods project a long-term warming of global air temperature of around 3°C (within an uncertainty range of 2°C to 4.5°C) in response to a doubling of the concentration of CO2 in the atmosphere.
Evidence from Earth’s past indicates that changes of this magnitude can have major long-term ramifications, such as sea level rise of many metres.
Continued increases in greenhouse gas levels are expected to lead to significant warming through the 21st century and beyond
Continued “business as usual” reliance on fossil fuels is expected to lead to a doubling of pre-industrial CO2 levels by about 2050, and possibly a tripling by about 2100. This emission pathway for CO2, coupled with rises in the other greenhouse gases, would be expected to produce a warming of around 4.5°C by 2100, but possibly as low as 3°C or as
high as 7°C.
If society were to shift rapidly away from using fossil fuels, there would be little reduction in the rate of global warming in the first couple of decades, but warming later this century and beyond would be significantly reduced (see Figure 5.1).
Climate models and basic physical principles indicate that global warming will generally be accompanied by increases in global-average humidity; more extreme hot events such as heat waves but fewer cold extremes; further decreases in the extent and thickness of Arctic sea-ice; shifts in rainfall (generally an increase in the tropics and high latitude regions and a decrease in the subtropics); further ocean warming; melting of mountain glaciers and polar ice sheets; and rising sea levels. Most of these impacts have already been observed (see Question 3).
Warming rates and other climate changes are not expected to be the same everywhere, due to changes in atmospheric circulation or other regional influences. Projections
of future climate for individual regions remain much less certain than global-scale projections. Different models often disagree, so definitive localised projections are not yet
This is particularly the case for regional rainfall projections. Some models also project substantial changes to phenomena such as El Niño or dramatic changes to vegetation. Many aspects of climate change will likely remain difficult to foresee despite continuing modelling advances, leaving open the possibility of climate change “surprises”.
Some climate change will continue for centuries, and some change will be essentially irreversible on a 1,000-year timescale.
Stabilisation of climate requires stabilisation of greenhouse gas concentrations. However, the inertia of the climate system, particularly the oceans and the ice sheets,
means that climate change will continue for centuries after greenhouse gas concentrations have stabilised.
Even if human societies completely ceased greenhouse gas emissions at some time in the future, atmospheric temperatures would not be expected to fall significantly for a
thousand years, as CO2 and heat are only gradually absorbed by the deep oceans. Sea level rise is also expected to continue for many centuries due to the ongoing melting of ice sheets and the gradual thermal expansion of the oceans in response to atmospheric warming.
Global warming above some threshold, believed to lie between about 2°C and 4.5°C, would lead to an ongoing melting of the Greenland ice sheet. If sustained for thousands of years, this would virtually eliminate the ice sheet, raising sea level by about seven metres. Most of the Antarctic ice sheet, by contrast, is expected to remain too cold for widespread melting.
It is possible that increased snowfall over Antarctica may partially offset other contributions to sea level rise.
In addition, accelerated outflow of ice has been observed from Greenland and West Antarctica. This is poorly understood, but could make these ice sheets more vulnerable
to future warming.
Reduction of greenhouse gas emissions could significantly reduce long-term warming
To have a better than even chance of preventing the global average temperature from eventually rising more than 2°C above pre-industrial temperatures, the world would need to be emitting less than half the amount of CO2 by 2050 than it did in 2000. To do this on a smooth pathway, global emissions (which are still rising) would need to peak within the next 10 years and the decline rapidly.
The Australian Academy of Science, which represents Australia’s foremost scientists, provides scientific advice to policy makers and promotes excellence in Australian science, has devoted considerable resources to untangling the science of climate change and presenting it in a simple and easily understood format.
The full report can be downloaded here for free.