Why has global warming stalled?

Solar energy is absorbed by the oceans

With Britain's heatwave reaching a peak, there could be no better moment to talk about why global warming has slowed to a standstill.

It reminds me of reporting on a drought a few years ago: while filming interviews with people about the impact, the heavens opened and rainwater was soon flowing down my neck.

So as journalists were invited to the Science Media Centre in London to hear how the worldwide rise in temperatures has stalled, the mercury shot up as if on cue to record the hottest day of the year so far.

In many ways, this event was long overdue: climate sceptics have for years pointed out that the world is not warming as rapidly as once forecast.

A lot depends on how you do the measurements, of course.

Each of the last few decades has been warmer than the last. But start your graph in 1998 - which happened to be an exceptionally warm year - and there hasn't been much global warming at all.

Gradually the words 'pause' and 'hiatus' which first featured in the blogs have crossed to the media and then to the scientists professionally engaged in researching the global climate.

The headline - which the scientists will not thank me for - is that no one is really sure why the rate of warming has stumbled.

No smoking gun

There are plenty of possible explanations but none of them adds up to a definitive smoking gun.

Industrialisation may lead to a drop in global temperatures in the 1940s

Professor Piers Forster of Leeds University has tried to quantify the different factors involved - what's known as their "radiative forcing".

Between 1998-2012, he reckons, manmade greenhouse gases were still the biggest influence, causing warming of 0.48 of a Watt per square metre (a key measure of energy flows to and from the planet).

At the same, he estimates, two other natural influences might have led to some cooling: a relatively quiet Sun might have been responsible for a reduction of 0.16 of a Watt/sq m and volcanic eruptions another 0.06 Watt/sq m.

A big unknown is the effect of aerosols - tiny particles released by industrial pollution which could cause a further cooling effect.

It is thought that the world's massive industrialisation after World War Two contributed to a slight drop in global temperatures in the late 1940s.

But the key factor - according to all the speakers at the briefing - is that whatever solar energy is making it through to the surface, much is being absorbed by the hidden depths of the oceans.

The Argo network of automated monitors has been deployed since 2005 to measure the waters as deep as 1,800m. This isn't a very long period but the data are apparently showing some warming - even in this short time frame.

And readings from satellites since 2000 show how much energy is arriving at the planet, and how much is leaving, so if the energy left behind is not manifesting itself in rising surface temperatures, then it must be going somewhere - and the deep ocean is the most plausible explanation.

Pauses expected

On top of that, the scientists say, pauses in warming were always to be expected. This is new - at least to me.

It is common sense that climate change would not happen in a neat, linear away but instead in fits and starts.

But I've never heard leading researchers mention the possibility before.

Arctic sea ice levels have fallen

Professor Rowan Sutton, of Reading University, said computer simulations or models of possible future climate scenarios often show periods of ten years with no warming trend - some even show pauses of 20-25 years.

And Professor Stephen Belcher, head of the Met Office Hadley Centre, said observations and models showed that on average there were - or would be - two pauses in warming every century.

I asked why this had not come up in earlier presentations. No one really had an answer, except to say that this "message" about pauses had not been communicated widely.

So where does this leave us, as greenhouse gases emissions keep rising but the temperature does not?

Dr Peter Stott, of the Met Office, pointed out that 12 of the 14 warmest years have occurred since the year 2000 and says that other indicators - like the decline in Arctic sea ice of 12.9% per decade and losses of snow cover and glaciers - still point to a process of manmade warming.

Bad maths

But what about another possibility - that the calculations are wrong?

What if the climate models - which are the very basis for all discussions of what to do about global warming - exaggerate the sensitivity of the climate to rising carbon dioxide?

Dr Stott conceded that the projections showing the most rapid warming now look less likely, given recent observations, but that others remain largely unchanged.

A Met Office briefing document, released at the briefing, says that, even allowing for the temperatures of the last decade, the most likely warming scenario is only reduced by 10% - so "the warming that we might have expected by 2050 would be delayed by only a few years".

Overall, it concludes, the pause "does not materially alter the risks of substantial warming of the Earth by the end of this century."

In other words, global warming is still on.

But until the pause can be properly explained, many people will take a lot of convincing - especially if the pause lasts longer than expected.