How much worse is a 4 degrees world?

ANALYSIS: risks linked to warming above 2C likely to be unpredictable and dangerous, says IPCC report 

(Pic: jthetzel/flickr)

(Pic: jthetzel/flickr)

By Gerard Wynn

A world which is four degrees hotter will face much bigger threats than half that warming, in crop damage, species extinctions and vanishing natural systems such as Arctic sea ice and coral reefs, a UN report found on Monday.

The report by the U.N.’s Intergovernmental Panel on Climate Change (IPCC) was the second in a three-part series documenting evidence, impacts and steps to cut greenhouse gas emissions.

Monday’s report focused on the impacts of rising temperatures.

One of the key findings was the scale of difference in impacts between 2 degrees and 4 degrees Celsius warming above pre-industrial levels.

Governments have agreed that the world should stay below 2 degrees warming to avoid more dangerous climate change, but are presently on track to exceed that.

Limiting warming to 2 degrees warming will require global cuts in global greenhouse gas emissions from the 2020s, the IPCC has said previously.

Such falling emissions would have to occur even as emerging economies such as China and India rapidly grew their economies, a de-linking which is almost without precedent.


The projection of future impacts is complicated by the use of different baselines. The IPCC sometimes compared future warming with pre-industrial levels, from 1850-1900, when global carbon emissions started to increase substantially.

It also sometimes compared future warming with “recent” levels, or the average temperature from 1986-2005.

Temperatures from 1986-2005 were 0.61 degrees Celsius warming than pre-industrial levels. Temperatures in 2012 were 0.85 degrees warmer than pre-industrial levels (1880).

Levels of 'additional risk' as a result of climate change (Pic: IPCC WGII)

Levels of ‘additional risk’ as a result of climate change (Pic: IPCC WGII)

The IPCC found that only moderate warming would lead to more extreme weather events and wipe out coral reefs and lead to the loss of Arctic sea ice in summer.

For example, warming of more than 1.5 degrees above pre-industrial levels (or less than twice the warming that has already occurred) would lead to the loss of 90% or more of all coral reefs.

Extreme weather events posed a “high risk” with an additional 1 degree warming above recent levels, or 1.6 degrees above pre-industrial levels.

“Extensive” species extinctions would follow additional warming of 3 degrees Celsius above recent levels, or 3.6 degrees above pre-industrial levels.

With more than 3 degrees warming above recent levels (3.6 degrees above pre-industrial), the potential for large and irreversible sea level rise took hold.
Beyond that, farmers worldwide would see serious crop damage.

“Global temperature increases of about 4°C or more above late-20th-century levels, combined with increasing food demand, would pose large risks to food security globally and regionally,” the report found.

In the longer term, impacts were more serious even at lower levels of warming.

For example, over the next 1,000 years or so, the whole Greenland ice sheet would melt at warming of 1 to 4 degrees above recent levels (1.6 to 4.6 degrees above pre-industrial), contributing up to 7 metres sea level rise, the IPCC said.


The IPCC’s summary report described climate risks by region, at 2 and 4 degrees warming above pre-industrial levels.

In Africa, the biggest risk was to crop production, risks described as “very high” with or without adaptation with 4 degrees warming, and “medium” for 2 degrees with adaptation. Adaptations included crop breeding and finance for smallholders to invest in fertilisers or irrigation.

In Europe, the biggest threats were in the Mediterranean, from reduced water availability and economic losses from heat waves including wild fires.

In Asia, the biggest losses were also from heat waves, which adaptation could only ease slightly at 4 degrees warming. In Australia the biggest risk was to coral reefs, which could not be protected at 4 degrees warming and faced high risks even with 2 degrees.

In North America the biggest threat was to natural ecosystems including forests from wild fires and water stress, where adaptations may not be able to cope with 4 degrees warming, and risks were also high with 2 degrees warming.

In South America, the biggest risk was to water availability, and in Polar Regions from melting ice and permafrost, which also impacted freshwater.

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