In 1990 the first assessment of the science of climate change undertaken under the aegis of the Intergovernmental Panel on Climate Change (IPCC) told the world that it should be worried about the increasing likelihood of climate change caused by greenhouse-gas emissions, but not that that change was already under way. Only in its second assessment, in 1995, did the IPCC conclude for the first time that humans were having a “discernible” influence on the climate—a finding which enraged fossil-fuel-funded lobby groups and led to an ugly campaign against one of the scientists involved.
By the tentative standards of the 20th century, the IPCC’s latest assessment of the physical science of climate change, published today, was a broadside. Human influence on the climate is now “unequivocal”: it is why the world is 1.1ºC (2ºF) hotter than it was in the late 19th century; it has moved jet streams, shrunk glaciers, stripped away Arctic sea ice, contributed to two decades of increased melting of the Greenland ice sheet, warmed the oceans and driven the past 50 years of sea-level rise. And these are just things about which the report’s authors are really sure—conclusions they call “extremely likely” and “virtually certain” or in which they express “high confidence”.
Much of the reason for this confidence is, unfortunately, experience. Again and again the report brings home that the reason why science can say so much more about climate change and its likely course now is that it has seen it actually going on—as, this week, the inhabitants of the eastern Mediterranean will be more than willing to testify.
But to think that the world has the full measure of climate change just because it is now in its grip would be wrong. The world does not yet fully understand what even the current 1.1ºC of warming really means. Since the Earth only just reached this temperature, most of what is being experienced now is, almost by definition, stuff that is quite commonplace once things get this hot. Much of what is unlikely-but-now-possible at 1.1ºC has yet to be seen. By the time the extremes of a climate 1.1ºC above the preindustrial have been experienced the temperature will have risen further.
Even under the most stringent emissions-reduction scenarios the IPCC thinks it is “more likely than not” that temperatures will exceed 1.5ºC above the preindustrial level within the next few decades. To stand a good chance of keeping the increase below 2ºC through emissions reduction would require the governments of the world to up their game a great deal. They would have to quickly set in place policies that would put their economies onto the emissions-reducing pathways they have pledged themselves to. Many would also have to up those pledges. The negotiations at the climate summit to be held under the UN Framework Convention on Climate Change this November will ring with demands that this be made to happen.
As the IPCC points out, the lower the total temperature rise can be kept, the less likely it is that the Earth will pass over various “tipping points”. But part of the lesson of seeing climate change already having an impact around the world is that it will continue for some time even after greenhouse-gas emissions stabilise, if or when they do. Sea-level rise will continue throughout the century. By 2100 more than half the tide gauges in the world will be registering high-water marks now experienced only once a century every year.
In terms of climate impacts, you ain’t seen nothing yet. That needs to be made true of action taken to constrain them, too.
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