Oceans cover more of the planet than anything else. So it makes sense that we need to know what’s happening to them to understand how humans are changing the climate.

If you follow climate science, you’d be forgiven for being a little confused recently by different news reports suggesting the oceans arewarming, slightlycoolingor doingnothing at all．

So are the oceans hotting up or aren’t they? And how does what happens beneath the waves influence what we feel up here on earth’s surface? Here’s our top to bottom look at the oceans and climate change.

More heat in, less heat out

Scientists have known for centuries that greenhouse gases in the atmosphere, such as carbon dioxide and methane, trap heat and warm the planet. This is known as thegreenhouse effect．

Scientists usesatellite measurementsto monitor how much of the sun’s energy enters earth’s atmosphere. A different set of measurements tells themhow much finds its way out again．

Thedifferencebetween those numbers is increasing, which means the earth istrapping more heatthan it used to. And that means the planetmust be warming．

A hiatus in surface warming

An interesting question is why temperatures at earth’s surface – that’s the air above land and the very top of the ocean – don’t always reflect what’s happening to the planet as a whole.

Over the last 15 years or so, surface temperatures have risenmuch slowerthan in previous decades, even though we’re emitting greenhouse gasesfaster than we were before．

This is what’s become known as the “hiatus”, “slowdown” or even “pause” in surface warming.

This raises an obvious question. If earth isgaining heatbut the surface isn’t warming very much, where is the heat going instead?

Where does the heat go?

Thescientific literatureisfull of discussionsabout where the extra heatmight be ending up．

We tend to be most interested in what temperatures at earth’s surface are doing, because that’s where humans live.

But actually only about one per cent of trapped heat stays in the earth’s atmosphere.

As the oceans absorb more than90 per centof the heat the planet traps, it makes sense to begin looking there for explanations of the changes we’re seeing at earth’s surface.

Looking to the ocean

Scientists have been taking the oceans’ temperature for centuries – from the days of dipping a thermometer into a bucket of water beside a ship, to the satellites that now encircle the globe.

The coloured lines below useall the data scientists have availableto show how thetotal amount of heatstored in the oceans changedbetween 1958 and 2009．

The blue lines are the surface down to 700 metres – this is generally referred to as the “upper ocean”. You can see a warming trend starting in about 1975, punctuated by short, sharp cooling after majorvolcanic eruptionsand the massiveEl Niñoin 1998.

But since about 2005, the pace of upper ocean warming has slowed quite a bit – especially in the top 300m, shown by the black lines.

How the amount of heat in the oceans changed between 1958 and 2009. Top 300m (black), top 700m (blue) and total depth (purple). Source:Balmaseda et al., (2013)

While warming has been slowing down in the upper ocean, it’s beenspeeding up in other parts．That means the oceans as a whole – shown by the purple line – are warming.

More heat is finding its way to thedeep oceaninstead of staying near the top. You can see this in the chart above – the gap between the purple line (whole ocean) and the blue line (top 700m) is wider now than it was in the early 2000s.

In fact, the data show the deep ocean – between 700m and 2,000m – is nowwarming fasterthan at any time in the last 50 years.

Why does it matter where the heat goes?

Winds constantly stir the upper ocean, keeping it very well mixed. Heat stored in the upper ocean passes easily into the layer of air above it, warming it up. This is what happened for most of the 1990s, causing surface temperatures torise very quickly．

Since the early 2000s, more heat has been pushed deeper into the ocean. That means the air above has warmed less,explainsthe University of Reading’s Professor Richard Allan:

“[M]ore of the heating due to rising greenhouse gas concentrations is being buried below the upper ocean layers, depriving the atmosphere of this extra ocean warmth.”

How do scientists know this?

Monitoring the seas becamea lot easierin about 2000 with the advent ofARGO– a network of free-floating buoys measuring temperature in thetop 2,000mof the world’s oceans.

As of 16th October 2014 there are around 3,600 free-floating ARGO buoys traversing the oceans monitoring temperature and salinity in the top 2,000m. The image above shows only buoys that reported in the previous 30 days. Source:ARGO programme

Argo has provided scientists with a wealth of new data,on top ofthe measurements from ships and satellites. But estimates of ocean warming still come with a fair bit of uncertainty.

There are few ARGO floats in the Arctic and Indian Oceans, for example. The Arctic is warmingalmost twice as fastas the global average and糟糕的数据there is likely to pull estimates of global average surface warming downby several tenths of a degree．

So it could be that the upper ocean has been taking up more heat than we thought. Not much data existed in the Southern Ocean before ARGO, which scientists think could meanwarming there has been underestimated．But since the data is imperfect, it’s hard to say whether warming right across the global ocean can fully explain the hiatus.

But the warming in the top 2,000m of the ocean in the past decadeagrees pretty wellwith the increase in the amount of heat being trapped by greenhouse gases in the atmosphere.

Into the abyss

Recent research suggests water below 2,000misn’t showing signsof warming. Below about 4,000 m –known as “the abyss”– the ocean may evencooling slightly．

But since ARGO floats don’t go deeper than 2,000m, reliable data that far down is scarce.

Scientists can use the fact that water expands as it warms to infer temperature changes at these depths. But estimating warming this way comes withquite a bit of uncertainty．

A new project, “Deep Argo“, hopes to rectify this, with a similar network of floats to monitor the ocean down to 6,000m deep.

Oceans apart

Knowing that heat is finding its way to the deep ocean is only part of the puzzle.

Scientists want to know why it’s happening, how long it will last and whether it’ll happen again in future.

Thetropical Pacificseems to hold the key. Trade winds have beenparticularly strongsince about 2000, whichscientists thinkcould be driving heat deeper and bringing cooler water up.

Scientists say this mechanism could mean earth’s average surface temperature is0.1 to 0.2 degreescooler than it would otherwise be, which is enough to account for much of the hiatus.

Not all scientists agree, however.Other researchsuggests a slow-moving fluctuation in the Atlantic is driving slower surface warming, rather than wind-related events in the Pacific.

The world’s oceans are all connected and unravelling their separate influences is complicated. While most scientists who work in this area suggest there could be anadditional role for the Atlanticin driving the hiatus, evidence for the Pacific Ocean mechanismis winning out．

How long will the hiatus last?

Scientists think the strengthening of the Pacific trade winds isdrivenby a larger natural cycle called the Interdecadal Pacific Oscillation (IPO) cycling from its warm to its cold phase.

Changes of pace in surface warmingaren’t unusual．地球表面温度记录充满了speed-ups and slow downs.Past hiatus periodshavecoincided withthe IPO’s cool phase.

Black curve is earth’s surface temperature with rising global average removed. Red and blue colours are PDO positive (warm) and negative (cool) phases. Source:Trenberth et al., (2014)

This implies the current hiatus will last until the IPO shifts back in the other direction. Professor Matthew England of the University of New South Wales concludes in arecent paper:

“这中断可能会持续下去的decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate”.

It follows that wecould see more hiatus periodsin the future each time the PDO flips to its negative phase. Likewise, we’d expect to see rapid warming during its positive phase.

But if the rate at which we’re emitting greenhouse gases continues to pick up pace, we’llbarely be able to detectthese hiatus periods in coming decades. That’s because the deep oceans may not absorb heat quickly enough to stop earth’s surface from warming, explains apaperby Nicola Maher of the University of New South Wales:

“Under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption.”

世界卫生大会t else could be slowing surface warming?

Scientific evidence is buildingthat the deep ocean is behind slower surface warming, and that the pace of surface warming will pick up again when the natural cycles that control it reverse. But a couple of other things may be contributing too.

Scientists think aslightly weaker sunin the 2000s compared to the 1990scould mean we’re seeingslightly less warming than in previous decades. An increase in the amount of particles in earth’s stratosphere that reflect sunlight, known as aerosols, may beplaying a parttoo.

But both of these effects are small. The drop in solar activity and higher aerosols mean surface temperature is about0.07 degrees coolerthan it would otherwise be.Other worksuggests the aerosol effect could bebigger, but ocean warming is still seen as the main player.

Oneimportant implicationof this would be that the hiatus in surface warming doesn’t mean carbon dioxide warms the planet warms less than scientists thought. If the heat is just finding its way to a different part of the climate system, climate sensitivityremains unchanged．

Rearranging heat

As the biggest absorber of heat, it make sense that understanding climate change should mean understanding the oceans.And the oceans aren’t an amorphous blue blob. Piecing togetherwhat’s happening at different depthsand in different parts of the globe is critical.

中断的可能是几个fa的产物ctors, each carrying its own uncertainties. Unravelling how different influences compare will take some time yet – if it’s possible at all.

There are still gaps in scientists’ knowledge of how the climate system behaves in response to greenhouse gases, and how natural factors alter that behaviour from one decade to the next.

But the bottom line is that understanding what’s happening to surface temperatures means focusing not just on the parts of the climate system we’re most familiar with, but also on how the earth rearranges heat out of sight.