Why 4 degrees will end the world as we know it: World Bank
The World Bank yesterday released a report prepared by the Potsdam Institute spelling out what the world is likely to experience if it warmed by 4 degrees — that’s looking increasingly likely by the end of the century without some serious policy changes by governments globally.
The report essentially attempts to summarise much of the research literature that has built up since the 2007 fourth IPCC Assessment Report and puts it in the context of the path we’re on unless we start taking this problem seriously. It doesn’t make for pleasant reading. World Bank President Dr Jim Yong Kim says “it is my hope that this report shocks us into action”, as he believes “a 4 degree world can, and must, be avoided”.
For the World Bank, with a primary purpose to help impoverished nations out of poverty, ”the lack of action on climate change not only risks putting prosperity out of reach of millions of people in the developing world, it threatens to roll back decades of sustainable development”.
The chart below provides an impression of the alternative emission paths and their likely implications for temperature rise. According to the report, we are on a path illustrated by the red line. However, if governments follow through on the pledges they’ve made at the UN’s Copenhagen and Cancun summits then it would put us on the purple line, giving a mean temperature rise estimate of 3 degrees.
But even this path still carries a 20% chance of temperature exceeding 4 degrees. If governments don’t make good on their pledges then the red line would imply a 40% chance of warming above 4 degrees.
Median estimates of temperature rise from probabilistic projections for several scenarios
The latest research, unlike older assessments, is more confident that land-based ice, and not just thermal expansion of existing sea water, will play a significant role in sea level rise. Older assessments were hopeful that increased snow falls might occur over Greenland and the Antarctic that could offset any melting from higher temperature. Unfortunately, the rate of land ice contribution to sea level rise has increased by about a factor of three since the 1972–1992 period. And both the Greenland and Antarctic ice sheets have been losing mass since at least since the early ’90s.
The chart below illustrates that our current emissions path (the red line) gives a mean estimate of a one metre rise in sea level by the end of this century. However, sea level would continue to rise substantially after this point. For example, even if global warming was limited to 2 degrees, global mean sea level could still rise by between 1.5 and 4 metres above present-day levels by the year 2300.
Probablistic projections of sea level rise for different emission scenarios
Rising temperatures will lead to increased likelihood of extreme weather events and heat waves. The report notes that the past decade has seen an exceptional number of extreme heat waves around the world. These events — Victoria’s 2009 heat wave and associated severe bushfire; Russia in 2010 (which claimed 55,000 lives); Europe in 2003 (70,000 premature deaths); the US in 2012 — were highly unusual with monthly and seasonal temperatures typically more than three standard deviations warmer than the local mean temperature for that period.
Another well understood feature of this warming will be a strengthening of the hydrological cycle because a warmer atmosphere can hold more water vapour. This tends to exacerbate droughts and flooding rains.
The table detailed below documents a series of extreme events over the past decade and the degree of confidence that these could be attributed to human-induced climate change. Many of these events are so outside the bounds of past experience that it seems there is a medium to high likelihood that global warming has contributed to the event’s severity.
Note: numbered references available from page 18 of the report
These extreme events, in conjunction with rising overall temperature, will take their toll on agriculture. While modelling prior to 2007 predicted some improvement in food production with warming of 1 to 3 degrees, according to the analysis, “research since 2007 is much less optimistic”. The report observes:
“These new results and observations indicate a significant risk of high-temperature thresholds being crossed that could substantially undermine food security globally in a 4°C world.”
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