Climate change facts and forecastsA guide to the climate science underpinning efforts to combat global warming, from the Intergovernmental Panel on Climate Change
Article at a glance
The ‘Physical Science Basis’ of the latest Intergovernmental Panel on Climate Change (IPCC) report spells out the expert consensus on climate change now and in the future. Here are the key findings.
Global warming “unequivocal”
The last 30 years has likely been the hottest period for 1400 years in the Northern Hemisphere, home to 90% of humanity. Since 1880, the global climate has warmed by about 0.85 degrees Celsius. Nine of the ten hottest years on record have occurred since 2000.
There was a slowdown in the rate of surface warming from 1998 to 2013 but the report describes this fluctuation as consistent with the long-term warming trend. Explanations include a weak solar cycle, the cooling effect of volcanic eruptions and heat transfer from surface to deep water in the oceans.
“People always pick 1998, but that was a very special year, because a strong El Niño made it unusually hot,” remarked Thomas Stocker, co-chair of the working group that produced the report, at the report’s launch. “If one chose 1999, the story would be very different.”
By 2100, Earth’s climate is forecast to be at least 1.5 degrees warmer. The worst-case scenario envisages 4.5 degrees of warming.
Humans the “dominant cause”
Climate scientists are 95% certain that burning fossil fuels, clearing forests, farming and other human activities are the primary driver of climate change. In 2001, they were only 66% certain of anthropogenic climate change. New evidence has reinforced the consensus view.
Human activities have led to the highest atmospheric concentrations of greenhouse gases for 800,000 years. The rate of increase is unprecedented in 22,000 years. We introduced 43% more ‘radiative forcing’ into the climate system in 2011 than in 2005. Over 90% of this extra energy went into the oceans, leading to rising seas, reduced sea ice and ocean acidification.
The Sun is not responsible for the recent warming, the report concludes, observing that fluctuations in solar energy reaching Earth “have not contributed to the increase in global mean surface temperature over the period 1986 to 2008”.
"Since the 1950s, many of the observed changes are unprecedented over decades to millennia,” the report notes.
The Arctic, for example, has warmed significantly more than the global average, leading to the dramatic disappearance of sea ice. The Arctic Ocean could be virtually ice-free in summer by mid-century, says the IPCC. Some experts are even more pessimistic (see 'Climate change report “complacent” says contributor').
Melting Arctic permafrost is also deeply concerning because it releases methane from the soil. Methane is over 20 times more potent than carbon dioxide over a 100-year period. By 2100, the IPCC forecasts permafrost will decrease by between 37% and 81% depending on emissions. The consequent methane releases could trigger runaway global warming, some argue, but because the science is not fully understood the IPCC does not report on this.
Sea levels rising faster
Seas are rising over 50% faster than was thought at the time of the last IPCC report in 2007. Global sea level rose 19cm between 1901 and 2010, largely due to water expanding as it warms, as well as melting ice sheets and glaciers.
Antarctica is losing almost five times as much ice annually as it did in the 1990s. The ice sheet melting in Greenland is even more severe, with losses leaping from 34 to 210 gigatons annually.
Mountain glaciers worldwide are losing more ice annually than in the 1970s and 1980s. Alaska, Canada, the Southern Andes and the Asian mountains are the worst affected. Excluding Antarctica, glaciers are forecast to shrink by 15% to 55% by 2100 if we reduce greenhouse gas emissions, but by 35% to 85% if we continue to increase emissions.
As a result, over 90% of coastlines will experience sea level rise this century. Even if CO2 emissions are stabilized, seas will continue rising for centuries. In the worst-case scenario average sea levels are forecast to be almost one meter higher by 2100.
More extreme weather “virtually certain”Weather extremes (see "What's with the extreme weather?")are becoming more common in particular regions, and more intense. It is “virtually certain that there will be more frequent hot and fewer cold temperature extremes,” the report says, as well as greater probability of longer-lasting heat waves, extreme heavy rainfall and longer monsoon seasons.
We should also expect more exceptional sea level rises such as the storm surge which inundated New York during ‘Superstorm Sandy’ in 2012. It is “virtually certain” that the frequency and intensity of storms in the North Atlantic has increased since the 1970s.
On the other hand, the 2007 report’s conclusions that anthropogenic climate change would likely lead to increased drought risk and severe dieback of the Amazon rainforest “are no longer supported”.
Nonetheless, the IPCC forecasts other regions like the Mediterranean and southwest US will experience increased drying if temperatures rise.
Climate change "challenges the two primary resources of humans and ecosystems, land and water. In short, it threatens our planet, our only home,” said Stocker.
Our carbon budget
For the first time the IPCC produced a carbon budget for humanity. To have a two-thirds chance of avoiding potentially catastrophic global warming of above 2 degrees Celsius we should limit total CO2 emissions to 1000 gigatons.
“Currently it is very clear we are not on a path that would respect that warming target,” warned Thomas Stocker. Having already emitted 532 gigatons, we are on course to exhaust our carbon budget by 2040.