Wednesday, July 10, 2013

No more global warming? Not so fast, says the WMO

It's true that the pace of global warming – as reflected by air temperature (a good long-term indicator but often misleading over periods less than 15 or even 20 years) – has slowed, even by some measures stopped, since the late 1990s.

A good example of how the impression of no warming has been hyped throughout the denialsphere is this graph produced last year by the conservative British tabloid, The Daily Mail:

It's this very graph that kept rearing its not-quite-sincere head in the B-roll “The Source” added to its interview with climate scientist Hans von Storch, which I described in a previous post.

People who seek out the big picture see something not quite so dramatic. Here is the full temperature record from the same source, the British Met Office's HadCRUT4 dataset:

You can see at a glance that temperature has been on a tear since about 1980. You can also see that periods of no warming – even cooling – are common since 1850 (when the record begins) and that even during the recent warming spurt, there are one or two short periods that you could “cherry pick” and isolate from the long-term trend if you wanted to make it look like warming had stopped.

Into this ambiguous picture comes the World Meteorological Organization. It's just released a report titled The Global Climate 2001-2010, A Decade of Climate Extremes. And that report also contains a graph or two – like this one:

What WMO's graph shows is that when you average the global air temperatures for each decade, you find that the most recent decade – 2001–2010, the decade when global warming “stopped,” by some accounts – actually warmed by a greater amount than any decade before it since 1881. Or as the WMO puts it: “... the decadal rate of increase between 1991-2000 and 2001-2010 was unprecedented.”

In a way, what the WMO's graph makes clear is what experts have been pointing out for years: lately, almost every year is among the hottest in the recorded history of our planet's temperature. To be exact, the ten warmest years have all occurred since 1998.

You can, if you like, look at the Daily Mail graph and say “global warming stopped in 1997.” Or you can look at the bigger picture and say “Since 1998, the planet has been experiencing its biggest heat wave in recorded temperature history. On the plus side, the temperatures have been pretty steady.”

So why do we even have these warming plateaus in the first place? It's no surprise to climate scientists given – for example – that over 90% of the planet's hot air is sucked up by the oceans, but with a highly irregular appetite. For a detailed discussion of what's likely driving the current plateau, check out “What has global warming done since 1998?”

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