Fossil fuel and land use behind CO2 rise

The sun rises over an industrial town
Greenhouse gases in the atmosphere now far exceed the levels before the industrial revolution. Photograph: Owen Humphreys/PA

The first volume of the fourth assessment report by the Intergovernmental Panel on Climate Change (IPCC) has been almost three years in the writing and brings together the work of 600 writers from 40 countries. More than 620 experts have reviewed the findings, and representatives of 113 governments have read and revised the key points.

The report assesses our current knowledge of climate change and the reasons behind it, looks at how the climate has already changed and how a range of different scenarios may have an impact in the future.

Story so far

According to the report there is evidence that the higher temperatures of the last half century are unusual compared with the at least the previous 1,300 years. As greenhouse gas levels have risen so have temperatures - global average air and ocean temperatures have been increasing and there has been widespread melting of snow and ice.

Eleven of the last 12 years have ranked among the 12 warmest years since records began in 1850, and as a result, the 100-year trend in temperatures has been adjusted upwards since the 2001 report, from an increase of 0.6C to 0.74C by the end of 2005. Much of the increase was recorded over the last 50 years, when the temperature increased by an average of 0.13C a decade - almost twice as fast as over the previous 100 years.

Research shows that the ocean has been absorbing more than 80% of the extra heat, causing water to expand and sea levels to rise. Between 1961 and 2003, the IPCC says, the global average sea level rose by an average of 1.8mm a year. And between 1993 and 2003 it was rising at a rate of 3.1mm a year. Whether this is a blip in the long-term trend, or an increase in the long-term outlook is unclear, but scientists are confident that sea levels rose much more quickly last century than in the 1800s.

Average Arctic temperatures have increased at almost twice the global average rate over the past 100 years, and the ice has shrunk by 2.7% each decade. Since 1900, the area covered by frozen ground has decreased by about 7%.

Elsewhere, more intense and longer droughts have been seen over wider areas since the 1970s, particularly in the tropics and sub-tropics, and in the North Atlantic there has been an increase in the incidence of typhoons and hurricanes.

The IPCC says the warming of the atmosphere and the ocean, together with the loss of ice, support the conclusion that it is very likely that climate change over the past 50 years is not down to natural causes alone and that it is extremely likely there were human causes.

The levels of greenhouse gases in the atmosphere have increased markedly since 1750 and now far exceed the levels before the industrial revolution. Human use of fossil fuels, and changes in land use, are the main drivers behind the increase in carbon dioxide levels, while agriculture has increased levels of methane and nitrous oxide.

The IPCC says levels of carbon dioxide increased from a pre-industrial level of 280 parts per million (ppm) to 379ppm in 2005 - far exceeding the natural range over the last 650,000 years, which has been measured as between 180ppm and 300ppm.

Most worryingly, the levels have increased more quickly over the past 10 years than they have since 1960 when scientists first began to record the increase.

What happens next

Scientists were able to draw from a larger number of simulations and models than when putting together the 2001 report - and the new data has caused them to suggest even more gloomy scenarios. It is now "virtually certain" that there will be fewer cold days and nights and more frequent hot days and nights over land.

Even if greenhouse gases were still at 2000 levels, the IPCC says over the next two decades global temperature rises of 0.2C a decade would be expected. With emissions set to rise its is "very likely" the increase will be twice that - 0.4C higher by 2030 - and that changes in the climate will be greater than those seen in the 20th century.

By the end of this century, temperatures could be between 1.8C and 4C higher than in 1999, although some of the scenarios considered by the IPCC suggested a rise of as much as 6.4C. In 2001, using a different methodology, scientists predicted an increase of 1.4 to 5.8C.

Warming of just 1.8C over the century could be achieved by moving rapidly towards a service and information economy, with reductions in the materials people use and the introduction of clean, efficient technologies, the IPCC says. There would have to be an emphasis on global solutions to economic, social and environmental sustainability.

The worst-case scenario, with warming of 4C, would be the result of rapid economic growth, a global population that peaks in the middle of the century and the rapid introduction of new technologies, but continued reliance on fossil fuels. A switch to non-fossil fuels would instantly cut the predicted temperature rise to 2.4C.

The scenarios for sea levels suggest a rise of between 18cm and 59cm over the same period, with the events that could lead to the highest temperatures also leading to the biggest increase in sea levels.

However, the predictions don't take into account the extra water that will enter the oceans as ice caps melt, so the real figures could be much higher. The last time the polar regions were significantly warmer than now was around 125,000 years ago, and the meltdown which that caused led to a 4-6m rise in sea levels.

If the ice flow in Greenland and Antarctica were to melt at a rate directly in proportion with the predicted rise in temperatures, the upper estimate for the sea level rise would increase by 10-20cm, and the IPCC says larger increases could not be excluded. It says knowledge of the impact of melting ice is "too limited" for a conclusion to be reached.

The report says snow cover will contract and sea ice will shrink in both the Arctic and Antarctic, and that late-summer sea ice may disappear entirely in the Arctic by the latter part of this century.

It is very likely that heat waves and heavy rain will become more frequent, and tropical cyclones will become more intense.

Because the oceans will be unable to take up carbon dioxide at the same rate as in the past, atmospheric greenhouse gas levels will increase more rapidly. To stabilise carbon levels at 450ppm, emissions would have to be reduced by around a third over the course of the century, the IPCC says.

The CO2 that has already been produced will continue to contribute to warming and sea level rise for more than a millennium.