?As We?re Going into the Future, the Average Circulation in the Atmosphere Will Probably Weaken?

How do you attribute the climate changes to human activities?

What we do is a fairly standard procedure, which is first to identify a signal in observed data and then to determine whether or not that signal is likely to have arisen from the internal chaotic nature of the system. We do that by re-sampling many very long climate model integrations, some of them as long as two thousand years. Once we assess that the signal is unlikely to have arisen from internal variability of the system, we then perform a set of experiments where we run the computer climate models - which are failry complicated programs that are run on super computers that represent the interaction between and the variations of the oceans, atmosphere, land surface and ice system. We are able to put on them the various contributions to climate forcing of the atmosphere such as greenhouse gases, aerosols, ozone changes. We also can include natural changes to the Earth budget coming from volcanoes and also natural changes coming from variations in solar forcing.

When we include all of the forcing we were able to reproduce the observed changes very well. Then we partitioned the forcing into two types: one is the natural variability which includes solar forcing and volcanic activity principally. With that one we were unable to reproduce the observed changes in the climate model. However when we forced the climate model with only the anthropogenic, that is the man-made, changes we were able to reproduce the observed changes. So we conclude that the changes are not likely to have resulted from natural variability nor natural forcing to the climate system, and are most likely explained due to the anthropogenic component of the forcing.

In your paper, you states that climate as a whole is slowly moving towards a more El Niño like state. What do you mean?

That is a very simple way to characterize what the wind changes look like. Many of the details actually, and this is one of the fascinating part for a climate scientist, many of the details aren’t exactly like El Niño. The wind changes look like El Niño, and traditionally, when we see something that looks like El Niño, we expect the system to look entirely like El Niño, but we’ve seen that the rain changes and, in particular, the changes of the ocean structures are not necessarily those that you get with El Niño. So it seems that some of the guidance that we have on how the climate system works, that we’ve gotten from understanding the year to year variations of the system, may not be necessarily translatable to understanding how the system in going to operate in the future.

You describe the evolution of the Walker circulation. What is the Walker circulation exactly?

The Walker circulation is the dominant East-to-West flow across the tropical Pacific. It was named after Sir Gilbert Walker, a British meteorologist. It is characterized by surface winds that blow from East to West across the Pacific, which pick up moisture from the ocean's surface as they go along and then these winds when they reach the west Pacific rise and release the moisture they’ve picked up in rains. So they provide heavy rainfalls to the west Pacific - eastern Indian ocean region such as Northern Australia, Indonesia, Malaysia, the Philippines and South-East Asia. Once this air has released the moisture, it then can move its way back to the East and radiating energy back to space. It becomes heavier, descend back to the East, and starts to loop again.

This Walker circulation slows down. What impact does it have on marine life?

The easterly trade winds, which are winds that blow from East to West across the equatorial Pacific, are the surface component of the Walker circulation, and are responsible for oceanic currents. In particular, currents that in the Northern atmosphere move towards the North and in the Southern atmosphere move towards the South. Those require the Ocean to replanish the near-surface with water from below. This water, because it’s outside of the region which has strong biological productivity, is generally nutrient-rich - so the winds bring the nutrient-rich water to the near surface where it can be used by the biology to produce the whole entire food web. When these winds weaken, the vertical circulation that is induced in the Ocean also weakens and so the transport of these nutrients from below could weaken and that could impact the biology.

What are your predictions for the 21^st century?

One of the interesting aspects of our results is that it gives us an assessment of the ability of our models. This slowdown of the Walker circulation was actually predicted by models a while ago, and now that we’ve seen it in the data we have enhanced confidence in the ability of our models to give us at least the order-one climate changes associated with global warming. So going forward in time, the climate models predict a continued slow down of the Walker circulation, an additional about 10% from what it slowed so far, and this is one of the most robust predictions of climate models. An interesting aspect of this: it is not just the Walker circulation that climate models predict to slow down, but generally all overturning atmosphere circulations in the Tropics. So it appears that, as we’re going to the future, the average circulation in the tropical atmosphere will probably weaken.

Can climate change be prevented?

Well, I would say that the climate system changes all the time. There’s a climate under every tree, under every leaf… and so it’s going to continue changing. The human contribution to it, the extent to which it can be controlled or not, is a very complicated issue that is beyond just pure climate science. It involves considerations in social science and other fields that really necessitate that whole different communities communicate with each other. So I cannot answer.

Gabriel Vecchi, thank you.

By Francesca Gilibert and Gilles Prigent

Gabriel Vecchi works at the NOAA/Geophysical Fluid Dynamics laboratory in Princeton, USA.