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Roz Pidcock

25.07.2013 | 12:45pm
ScienceTemperatureTaking Earth’s temperature: Three Met Office reports examine the warming pause, climate sensitivity and taking a broad view of climate change
SCIENCE | TEMPERATURE | July 25. 2013. 12:45
Taking Earth’s temperature: Three Met Office reports examine the warming pause, climate sensitivity and taking a broad view of climate change

Much of the debate around climate change taking place in the media, online and in the world of campaigning focuses on the part of the planet where humans live – the surface. We are warned about climate change in terms of temperature rise – two degrees is ‘dangerous’ – and when temperatures don’t rise steadily, some claim that “global warming has stopped”.

Last week saw a piece on the BBC’s Sunday Politics which focused on a graph showing surface temperature from 1980 to argue global warming had “plateaued”, suggesting that climate scientists were at a loss to predict or explain it.  

With impeccable timing, the Met Office has just released three useful reports on this very topic, which explore some reasons why looking only at surface temperatures might mean you’re missing the broad picture.

Taking earth’s temperature

The three reports, published this week, have a lot of interesting detail and are worth a read in full.

The first looks at what different measurements of the earth’s system can tell us about climate change. The second looks at what’s causing surface warming to slow, and the third looks at whether the slowdown affects projections of substantial warming by the end of the century.

On surface temperatures the Met Office says:

“Global mean surface temperatures rose rapidly from the 1970s, but have been relatively flat over the most recent 15 years to 2013.”

A broader look at climate change considers other indicators of how the climate is changing, the reports say. One pretty clear indicator is Arctic sea ice. The report shows the following graph and explains how the amount of ice – particularly in summer – has continued to decline:

“Over the last decade, while global surface temperature rise has paused, the Arctic land areas have continued to warm markedly … The record low summer Arctic sea ice extent observed in 2012 represents a 50 per cent reduction in ice cover compared to the 1980s”.

On observations of glaciers, which globally have shrunk every year for 22 years in a row, the report says:

“[T]he continuing retreat of most of the world’s glaciers is one of the clearest signals of ongoing climate change.”

The report also explains how new measurements from the ARGO network of ocean buoys are beginning to offer new insights into how the ocean temperatures are changing. And that has a lot to do with what’s causing surface warming to slow, it says – the topic of the second report.

Pause for thought

The amount of the sun’s energy reaching earth hit a pronounced low in recent years as part of an 11-year cycle, but the drop is not enough to account for the surface warming slowdown, the report says.

When volcanoes erupt they spit reflective particles into the atmosphere, but the cooling effect drops off after two to three years, according to the report. No major eruptions recently means volcanic activity can’t explain the slowdown in surface warming either.

In earth’s recent past, other natural fluctuations in the climate have caused several periods with little surface warming, or even cooling. The Met Office suggests two such periods lasting about ten years are expected on average each century – and that in that respect the current slowdown isn’t unusual. However, it does say that a period lasting more than 20 years is unlikely unless something other than natural variability is contributing too.

The report suggests that while the land’s surface has continued to warm in the last decade, the slowdown in surface temperature is at least in part due to how the oceans take up heat.

Data from the ARGO floats traversing the world’s oceans indicate the surface of the oceans hasn’t warmed much in the last decade, while warming below 300m has increased. That means the total heat absorbed has continued to rise, shown by the blue line in the graph below.

Met Office _oceanheat

The report highlights some caveats to this. Ocean measurements don’t exist over a long enough period of time or a large enough volume of the ocean to prove the theory definitively. For example, how much the ocean below 700 m is warming is less clear – as there are fewer measurements there.

But the report says the evidence scientists have so far indicates heat is going into the deeper ocean instead of staying at the surface.

Importantly, the Met Office suggests this redistribution of heat isn’t random but probably caused by changes in ocean circulation, with the Pacific Ocean playing a key role. At some point natural cycles will reverse and the oceans will release that heat, causing surface temperatures to rise again:

“[T]he heat in the ocean is merely being rearranged; it is effectively ‘hidden’ from the surface only to re-emerge at some later date.”

Slow changes in ocean circulations are a good reason for looking at more than just a decade to determine how temperatures are changing, the report says:

“[V]ery little can be concluded from ten-year trends with respect to global warming â?¦ It is only with averaging periods of 30 years or longer that climate change can be detected robustly.”

Does the hiatus mean we’ll see less warming overall?

The third report addresses whether the recent period of slower surface warming should make scientists lower their assessment of how sensitive the climate is to greenhouse gases – something called climate sensitivity.

On this, the Met Office says:

“[T]he prospects of substantial global warming by the end of the century are not materially altered by the recent pause in global surface warming.”

That’s largely because even with the slowdown in surface warming, the average temperature for last decade was still the warmest on record, as the graph below shows.

Met Office _Decadalaverages

But while the amount of warming in the long term doesn’t seem affected, the heat from the oceans might take a while to be released – which means how much of that warming we’re likely to see in the next few decades might come down a bit. The report says:

“The most likely warming is reduced by only ten per cent, indicating that the warming that we might previously have expected by 2050 would be delayed by only a few years.”

Some scientific literature suggesting possible lower climate sensitivity is being considered by the Intergovernmental Panel on Climate Change (IPCC), so it will be interesting to see how the IPCC’s assessment compares with the Met Office’s when it is published in September.

Understanding how greenhouse gases are affecting the planet means understanding more than how surface temperatures are changing. As the Met Office puts it:

“[C]onsiderations of our climate and its long-term change must continue to be made on the basis of robust, holistic assessments – including, but not limited to, global mean surface temperature – to reflect the fullest possible picture of our changing climate .”

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