IPCC climate change facts: Humanity is responsible
A guide to the climate science underpinning efforts to combat global warming, from the Intergovernmental Panel on Climate Change
September 30, 2013
Article at a glance
- Earth could be 4.5 degrees warmer by 2100
- Scientists 95% certain humanity responsible for climate change
- Ice sheet, glacier melt point to faster sea level rise
- Stronger evidence on global warming link to extreme weather
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”.