Over the last twelve years, global average surface air temperature has plateaued and there’s a lot of confusion about why this has occurred because during the same time, greenhouse gasses in the atmosphere have shot up to record levels. This pause in the surface warming doesn’t mean that global warming has stopped at all. We see Arctic Sea ice melting to record low levels. The land ice sheets right around the world are melting rapidly. Ocean temperatures continue to warm. So it’s not a contradiction of global warming, but it has confused scientists because we do expect the surface temperatures around the world, to progressively warm decade by decade. Having seen the hiatus in surface warming, a lot of scientists have been trying to understand exactly what’s causing it.
We know that of all of the energy trapped by greenhouse gasses over the last century, already over 90% of that has ended up in the oceans. So all we need is an extra acceleration of ocean heat uptake, and we can get these hiatuses that last for a decade or more. We took all the available measurements of climatic conditions over the last twenty years and we used those to estimate how much extra heat is going into the ocean, and what we found is that there was more than enough heat going into the ocean to account for that plateauing of global surface temperatures, in spite of this increase in greenhouse gas. The reason the oceans are taking up extra heat (and this is particularly occurring in the Pacific Ocean), is that the trade winds have strengthened considerably over the last two decades. In fact, the trend over the last two decades is unprecedented in the measurement record and it’s way beyond the strength of the trends ever captured by climate models. So over the Pacific Ocean, the trade winds blow from the Americas across towards the West Pacific. When they do so, they take all of the surface water westward so a lot of the warm surface water ends up North of Australia in a big pool of water we call the West Pacific Warm Pool.
Now if the trade winds blow particularly strong, that warm water that’s piled up there, starts to converge into the ocean interior. There are overturning cells basically taking warm surface water and pumping them into the interior of the ocean and in a way it’s locking away energy that we’ve obtained from greenhouse gas into the subsurface ocean, and that’s what causes this hiatus. But if the trade winds slow down or stop, what happens is that heat absorption by the ocean reverses and we actually get heat coming back up into the atmosphere, and the bad news is that out of this hiatus we’re expecting quite rapid warming to occur. .