According to the Australian Academy of Science, "The Earth’s climate has changed. The global average surface temperature has increased over the last century and many other associated changes have been observed. The available evidence implies that greenhouse gas emissions from human activities are the main cause. It is expected that, if greenhouse gas emissions continue at business-as-usual rates, global temperatures will further increase significantly over the coming century and beyond."
THE CARBON CYCLE
According to the Australian Academy of Sciences:
Large amounts of CO2 are continually transferred to and from the atmosphere, which exchanges carbon with the oceans and vegetation on land. Until around 200 years ago, these natural exchanges were in rough balance, shown by the nearly constant concentrations of atmospheric CO2 for most of the last two thousand years.
The importance of human-caused CO2 emissions is that they are disturbing this balance, adding carbon to the atmosphere faster than it can be removed by uptake by vegetation, the slow mixing of CO2 into the deep oceans, or the even slower weathering processes that control the carbon balance on geological timescales.
According to the NASA Earth Observatory:
Carbon is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilizations-our economies, our homes, our means of transport-are built on carbon. We need carbon, but that need is also entwined with one of the most serious problems facing us today: global climate change.
Forged in the heart of aging stars, carbon is the fourth most abundant element in the Universe. Most of Earth's carbon-about 65,500 billion metric tons-is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels.
Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth.
Over the long term, the carbon cycle seems to maintain a balance that prevents all of Earth's carbon from entering the atmosphere (as is the case on Venus) or from being stored entirely in rocks. This balance helps keep Earth's temperature relatively stable, like a thermostat.
This thermostat works over a few hundred thousand years, as part of the slow carbon cycle. This means that for shorter time periods-tens to a hundred thousand years-the temperature of Earth can vary. And, in fact, Earth swings between ice ages and warmer interglacial periods on these time scales. Parts of the carbon cycle may even amplify these short-term temperature changes.
On very long time scales (millions to tens of millions of years), the movement of tectonic plates and changes in the rate at which carbon seeps from the Earth's interior may change the temperature on the thermostat. Earth has undergone such a change over the last 50 million years, from the extremely warm climates of the Cretaceous (roughly 145 to 65 million years ago) to the glacial climates of the Pleistocene (roughly 1.8 million to 11,500 years ago).
DEFORESTATION
According to CSIRO's Dr. Pep Canadell, from the Global Carbon Project and CSIRO Marine and Atmospheric Research:
"Deforestation in the tropics accounts for nearly 20 per cent of carbon emissions due to human activities," Dr Canadell says. "This will release an estimated 87 to 130 billion tonnes of carbon by 2100, which is greater than the amount of carbon that would be released by 13 years of global fossil fuel combustion. So maintaining forests as carbon sinks will make a significant contribution to stabilising atmospheric greenhouse gas concentrations."
"The new body of information shows considerable value in preserving tropical forests such as those in the Amazon and Indonesia as carbon sinks, that they do not release the carbon back into the atmosphere as has been suggested,"
"However, it also demonstrates the need to avoid higher levels of global warming, which could slow the ability of forests to accumulate carbon."
CSIRO Fast facts:
- Deforestation in the tropics accounts for nearly 20 per cent of carbon emissions due to human activities
- This will release an estimated 87 to 130 billion tonnes of carbon by 2100, which is greater than the amount of carbon that would be released by 13 years of global fossil fuel combustion
- Reducing deforestation is just one of a portfolio of mitigation options needed to reduce concentrations of greenhouse gases in the atmosphere
- The world's established forests remove 2.4 billion tonnes of carbon per year from the atmosphere - equivalent to one third of current annual fossil fuel emissions - according to new research published today in the journal Science.
BURNING FOSSIL FUELS
According to the Australia Academy of Sciences:
There has been a recent acceleration in the growth rate of CO2emissions from fossil fuels and industrial sources. From 2000 to 2007 these emissions grew by 3.5% per year, exceeding almost all assumed scenarios generated in the late 1990s.
This pulse of CO2 emissions growth coincided with a period of rapid global economic growth. There will be a small, temporary downturn in CO2 growth, associated with the 2008-09 global financial crisis.
It is very likely that most of the recent observed global warming is caused by increasing greenhouse gas levels. It was predicted more than a century ago that increases in CO2 would act like added trapping more heat near the surface. This extra CO2 was also predicted to make the stratosphere colder.
Satellite measurements over recent decades have confirmed the extra insulating effect not only of CO2, but also of each additional greenhouse gas. Moreover, trends over the last 40 years, superimposed on natural year-to-year variations, have been observed which show that the upper atmosphere has cooled and the surface of the Earth and the lower atmosphere have warmed significantly.
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 then decline rapidly.
The projected consequences of additional levels of greenhouse gases depend on future scenarios of greenhouse gas emissions.
