Oceans apart

17 June 2017

9:00 AM

17 June 2017

9:00 AM

Readers of The Spectator will be familiar with the argument that climate change, like Britpop, ended in 1998. Raised on a diet of Matt Ridley and James Delingpole, you may have convinced yourself that climate scientists, for their own selfish reasons, continue to peddle a theory that is unsupported by real-world evidence.

You may also have picked up the idea that the ‘green blob’, as it has been called in these pages, is somehow suppressing the news that global warming is a dead parrot. That was the case made by Dr David Whitehouse, science editor of Lord Lawson’s Global Warming Policy Forum in a Spectator blog in February last year. He accused the world’s media of ignoring a paper in Nature Climate Change which concluded that the rise in global surface temperature had stalled, contrary to the narrative of man-made climate change. In contrast, an earlier paper by the National Oceanic and Atmospheric Administration (NOAA) in Science magazine, which questioned the existence of that hiatus, had been given huge coverage.

Those of us who work in climate research do not, of course, ignore evidence. A study published in Nature Climate Change does not go unnoticed. But the particular paper to which Whitehouse referred does not counter the reality of man-made change. By the time he wrote his piece, the hiatus in global air temperatures had already come to a blistering halt. The years 2014, 2015 and 2016 were the three hottest years on record — an unprecedented run.

But this is only part of the story. Anyone who considers climate change to be all about air temperatures at the Earth’s land surface misses something rather important. The evidence is not just blowing in the wind; it is 500 fathoms deep.

As a land species, it’s hardly surprising that we’re more concerned about what’s going on in the atmosphere than with conditions under the sea — but in the context of global warming that’s a big mistake. Around 93 per cent of the extra heat gained by the Earth over the past 50 years has sunk into the ocean, while 3 per cent has made ice melt, and 3 per cent has warmed the land. Only around 1 per cent has stayed in the atmosphere. So if we just measure air temperatures, we’re looking in the wrong place for climate change. Recent analyses by the World Meteorological Organisation and independent researchers have looked at deep-ocean as well as sea-surface temperatures, and both groups found that significant increases in total ocean heat content began around 1980, continuing more rapidly after 1998.

Not all the heat which is absorbed by the ocean stays there. Changes in circulation in the Pacific involve warm water shifting towards South America, raising air temperatures as it does so. Such El Niño events have contributed to the sharp rise in global air temperatures over the past three years.

The apparent slowdown in global temperature rise in the early years of this century was nothing more than the Earth’s climate system expressing its natural variability. Like the weather in London, the Earth’s climate is fickle: what we see in the climate from year to year is much like what we see in the weather from day to day, or week to week. The years between 1998 and 2013 were the equivalent of a spell of cool weather following a heatwave. Yet all the while, taking air and ocean heat content combined, the Earth was warming. Now that the most recent El Niño event has ended, global air temperatures ought to be falling, but they aren’t. The world saw its third hottest January ever, followed by the second hottest February, March and April. The atmosphere and the ocean are warming in tandem, as predicted by climate models.

It is not easy to measure how much extra heat has entered the ocean as a result of human influences on the climate. Given that seawater is around 1,000 times as dense as air, small increases in water temperature represent a huge amount of heat being absorbed. It’s tough to demonstrate a whole-ocean average temperature increase of less than 0.1°C in about 1.4 billion cubic km of seawater. Tough, but not impossible — steadily, scientists have managed to complete the picture. Four years ago, the Intergovernmental Panel on Climate Change concluded that such ocean warming was ‘virtually certain’. Following new findings of recent weeks and months, the qualifier ‘virtually’ is now unnecessary, putting to bed any contention that global warming ended in 1998 — it is just that for a while the main effect was on water, not air, temperatures.

What happens in the ocean matters, because rising sea temperatures reinforce climate change in several ways. Warmer sea water can release methane trapped on the sea floor. Some of it finds its way to the surface and into the atmosphere, where it acts as a greenhouse gas, trapping heat — at least 30 times more effectively than carbon dioxide. Warmer water also means less sea ice. That matters, because ice reflects the sun’s rays. With less sea ice, the ocean will absorb even more heat. As the ocean warms, it expands, lifting coastal ice-shelves and making it easier for glaciers to slip into the sea. New analyses now suggest that sea levels could rise by up to a metre, and maybe more, in our children’s lifetimes.

Genuine scepticism can be constructive, since science responds to challenges by obtaining new evidence to test ideas. But those who summarily dismiss evidence when it has become overwhelming no longer deserve the name sceptics — it’s then out-and-out denial. There is no hoax; scientists like me gain nothing from exaggeration.

Yet the worst-case scenarios are not inevitable. They can be averted by action to reduce, and eventually end, greenhouse gas emissions. While Donald Trump and others might dismiss inconvenient truths, science is now in no doubt that the planet is warming, and that there is a need to take action on a worldwide basis. The Paris agreement will be the future, whereas the so-called global-warming hiatus is already history.

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