Climate facts and predictions from the IPCCThe basic science and forecasts from the fifth Intergovernmental Panel on Climate Change (IPCC) report by leading climate scientists.
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Here is the critical information and projections on climate change as outlined by ‘The Physical Science Basis’ of the IPCC’s Fifth Assessment on climate change.
Global warming is “unequivocal” and climate scientists are 95% certain that humans are the “dominant cause”. New evidence has increased this level of certainty from 90% in the last report in 2007.
"Since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased," notes the report.
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, it says that these “have not contributed to the increase in global mean surface temperature over the period 1986 to 2008”.
Greenhouse gases carbon dioxide, methane, and nitrous oxide in our atmosphere have increased since pre-industrial times by 40%, 150%, and 20% respectively, due to fossil fuel use, deforestation, land use change and agriculture. These rates of increase are unprecedented in the last 22,000 years.
The massive recent increase has led to a rise of average temperatures by 0.85 degrees Celsius since 1880. Scientists say it is likely that the last 30 years has been the hottest period in the Northern Hemisphere for 1400 years.
The vast majority of that extra energy – over 90% between 1971 and 2010 – has gone into the oceans, leading to reduced Arctic sea ice, sea level rise and ocean acidification.
Even if we were to significantly reduce, or ‘mitigate’, carbon emissions and thereby limit CO2 levels to 421 parts per million by 2100 – in 2013, they topped 400ppm – we are likely locked into global temperature rise of 1.5 degrees by 2100. The worst-case high emissions scenario (1313ppm by 2100) foresees up to 4.5 degrees of warming.
Warming will not be equally distributed. Impacts will be more pronounced in northern latitudes. Arctic temperatures have increased faster than global average temperatures. By mid-century, the Arctic might well be ice-free in summer. Since 1979, it has lost 9.4% to 13.6% of summer ice per decade
Meanwhile, permafrost is also on the retreat in the Arctic. This has freed large amounts of methane, a greenhouse gas far more potent than carbon dioxide. By the end of the 21st century, the area of permafrost near the surface (upper 3.5 m) is projected to decrease by between 37% (low emissions scenario) to 81% (very high emissions scenario). 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.
Meanwhile glaciers all over the world are diminishing in volume. Excluding Antarctica, glaciers are expected to diminish in volume by up to 55% for the mitigation of emissions scenario and up to 85% for the very high emissions scenario by the end of the century.
Sea level rose 19 centimeters during the period 1901 to 2010, due largely to the fact that warm water has a larger volume than cold water, and also melting ice sheets and glaciers. Further sea level rise by the end of the century is projected to be up to 0.55m for the low emissions scenario and up to 0.98m for the very high emissions scenario. 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 also notes that weather extremes are becoming more common in particular regions, and also more intense. It is “virtually certain that there will be more frequent hot and fewer cold temperature extremes” and thinks it likely that there will be longer-lasting heatwaves in future as well as extreme heavy rainfall and longer monsoon seasons in some places.