Humankind's emissions of greenhouse gases are breaking new records every year. Hence we're on a path towards 4-degree global warming probably as soon as by the end of this century. This would mean a world of risks beyond the experience of our civilization - including heat waves, especially in the tropics, a sea-level rise affecting hundreds of millions of people, and regional yield failures impacting global food security.

These are some of the results of a report for the World Bank, conducted by the Potsdam Institute for Climate Impact Research (PIK) and Climate Analytics in Berlin. The poorest in the world are those that will be hit hardest, making development without climate policy almost impossible, the researchers conclude.

"The planetary machinery tends to be jumpy, this is to respond disproportionately to disruptions that come with the manmade greenhouse effect," PIK's director Hans Joachim Schellnhuber points out. "If we venture far beyond the 2-degree guardrail, towards 4 degrees, we risk crossing tipping points in the Earth system."

This could be the case with coral reefs which face collapse under unabated global warming, or with the Greenland ice sheet. To melt the ice sheet would take thousands of years, yet this might be an irreversible process that could start soon. "The only way to avoid this is to break with the fossil-fuel-age patterns of production and consumption," says Schellnhuber.

Climate impacts: Heat waves, sea-level rise, yield failures
Already today impacts of climate change are observed. The Russian heat wave in 2010, according to preliminary estimates, produced a death toll of several thousand, annual crop failure of about 25%, and economic losses of about US$15 billion.

Extreme events like this at 4 degrees global warming would become "the new normal" in some parts of the world, according to the report. In the tropics, the coolest months at the end of the century are likely to be substantially warmer than the warmest months today.

Sea level, under this scenario, would rise by 50 to 100 centimeters within this century, and more so in coming centuries. The rate of rise varies from one region to the other, depending on sea currents and other factors. Projections suggest that sea-level rise will be strongest in countries like the Philippines, Mexico, and India.

Within economic sectors, too, tipping effects with rapidly increasing damages can occur, for instance in agriculture. Already, observations showed that important cereals are sensitive to temperature increases passing certain thresholds, resulting in large-scale yield failure. Changes in the water cycle can aggravate this, when droughts occur or flooding affects farmed land.

World Bank President Kim: "A 4-degree warmer world can, and must be, avoided"

"The report draws from the current state of science and delivers new analysis of heat waves and regional sea-level rise, so of course there remain some uncertainties," says William Hare, co-founder of Climate Analytics in Berlin and guest scientist at PIK.

"We work with that by defining risk as potential damage multiplied with the probability - a rather improbable event can be a great risk if its impacts are huge."

World Bank President Jim Yong Kim who was nominated early this year by US President Barack Obama and assumed his new position in July, has personally been briefed on the 4-degrees report by Schellnhuber some weeks ago in Washington D.C..

"A 4-degree warmer world can, and must be, avoided - we need to hold warming below 2 degrees," President Kim now said in a statement.

"Lack of action on climate change threatens to make the world our children inherit a completely different world than we are living in today. Climate change is one of the biggest single challenges facing development, and we need to assume the moral responsibility to take action on behalf of future generations, especially the poorest."