Facts and predictions from the IPCCThe fourth Intergovernmental Panel on Climate Change (IPCC) report from leading climate scientists concluded with 90 percent certainty that human activity is ...
First, the facts as outlined by the report.
Global warming is a reality and “very likely” human-induced. Although the term “very likely” may seem vague, it is as close as 700 scientists, 2,500 reviewers, and countless government officials can get to consensus about whether humanity is to blame.
"Numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones," notes the report.
Greenhouse gases like carbon dioxide in our atmosphere have increased since 1750 due to the consumption of fossil fuels, new forms of land use, and agriculture.
While atmospheric pollution had a cooling effect during previous centuries, the massive recent increase has lead to a rise of average temperatures by 0.74 degrees Celsius since 1901. Scientists are 90 percent sure that the second half of the 20th century has been the hottest period in the Northern Hemisphere for 500 years.
A doubling of carbon dioxide levels in the atmosphere equates to a surface warming of some 3 degrees Celsius plus or minus one degree. Even if we were to reduce carbon emissions to year-2000 levels, such a doubling of carbon dioxide is unpreventable.
Warming, the report adds, will not be equally distributed. Impacts will be more pronounced in northern latitudes. Arctic temperatures have increased twice as fast as global average temperatures. Summer ice in the Arctic Ocean is declining by 7.4 percent per decade. By the end of the century, the Arctic might well be ice-free in summer.
Meanwhile, permafrost is also on the retreat. Since 1900, the seasonally frozen ground in the Northern Hemisphere has shrunken by some 7 percent. This has freed large amounts of methane, a greenhouse gas far more potent than carbon dioxide.
Massive releases of methane could trigger runaway global warming, some scientists believe, but these kinds of amplification effects are not yet properly understood and so the IPCC report does not consider them.
Climate change skeptics have suggested that changes in the sun’s radiation account for global warming. Although the IPCC scientists have found fluctuations in the sun's radiation, the effects are nearly 20 times weaker than human-induced warming.
Scientists say that sea levels rose 17 centimeters during the 20th century, most of it due to the simple fact that warm water has a larger volume than cold water.
Meanwhile glaciers all over the world are retreating, an effect that is also perceivable at the fringes of the vast Antarctica ice shield. With the melting of icecaps and glaciers, the annual sea level rise has nearly doubled since 1993 to a rate of about 3.1 mm.
Even if carbon dioxide emissions can be stabilized, sea levels will keep on rising for centuries until the temperature gain reaches the ocean depths.The IPCC findings also show that the atmosphere now holds more water vapor, one of the driving forces behind tropical storms and floods.
The Atlantic has been particularly affected by more frequent and severe tropical cyclones, a phenomenon in line with rising surface water temperatures. Since the 1960s, westerly winds have gained in strength all over the planet. The report says that there is a sixty percent chance that recent severe storms have been boosted by global warming.
Precipitation patterns, too, changed over the last century. There is significantly more rain in the eastern parts of North and South America, northern Europe, and northern and central Asia. On the other hand, dry spells are more frequent in the Sahel, the Mediterranean, southern Africa and parts of southern Asia.
The IPCC Scenarios
The world’s leading scientists have devised seven climate scenarios for the 21st century. It all depends, they say, on the level of demographic and economic development, and how serious we are about the fight against global warming.
The Benchmark: If we manage to stabilize our greenhouse gas emissions at the levels of the year 2000, we will still feel the heat, but the increase will be less than 1 degree Celsius over the next one hundred years. Unfortunately, this option is not even considered a real scenario but rather a benchmark to compare with more realistic models.
Best Case Scenario: Scenario B1 presents the most optimistic outlook: by mid-century, the global population will hit a peak and decline thereafter. Rapid economic changes will bring about a service and information economy based on clean and efficient technologies.
The international community will unite around policy solutions—such as the Kyoto Protocol—for the reduction of greenhouse gases. While all this sounds promising, global warming will still occur, albeit not beyond a range of 1.1 to 2.9 degrees Celsius. Sea levels rise between 18 and 38 centimeters by the end of the century.
Population Pressure: Scenario B2 is less rosy: the global population will constantly grow while climate change mitigation efforts have a regional focus. This translates into a temperature rise of some 1.4 to 3.8 degrees Celsius. Sea levels increase some 20 to 40 centimeters by 2100.
The so-called A1 scenario is split up into three. Each sub-scenario is based on rapidly growing economies and a growing population, although populations will decline towards the second half of the century.
Business as Usual (A1F1): a world that still runs on coal and gas. It is here that predictions are most shocking: temperature gains of some 2.4 to 6.4 degrees are plausible. The sea would rise some 26 to 50 centimeters by the end of the century, flooding large coastal cities and numerous islands.
Balanced Development (A1B): the most probable scenario given current trends, is also alarming. While fossil fuels are still widely used, they are part of a more balanced energy mix. Still, by the end of the century, temperatures will have risen some 1.7 to 4.4 degrees Celsius, with the oceans gaining some 21 to 48 centimeters.
Rainfall is likely to decrease by some 20 percent in the subtropics, while more rain will fall in the northern and southern latitudes. The Gulf Stream will not stop, but it will lose about a quarter of its force.
Industrial Revolution (A1T): a world that has lived through a third industrial revolution—a widespread conversion to “green” energy sources. It is similar to B1 in the sense that temperatures and oceans will rise, but to an extent that experts call “manageable".