Making sense of the slowdown


The Earth’s climate is controlled by the energy balance at the top of the atmosphere. If more heat enters the atmosphere than leaves, then the planet warms. Adding heat trapping gases changes the balance, which in turn causes warming. Ocean heat measurements show that the planet is indeed absorbing heat. Despite this fact, it is often claimed that the global warming has stopped. This claim is inspired by evidence that warming of the atmosphere has been slower over the past one and a half decades. This slowdown is sometimes called the hiatus. However, there are other factors which affect the atmosphere over shorter periods. These can cause faster or slower warming of the atmosphere. To understand the slowdown in warming, we need to understand some of these factors.

If we look at the global surface temperature over the past 3 decades, there are big changes in temperature from year to year. We know the cause of some of these variations. One of the biggest is the El Nino cycle. El Nino is a phenomena in which heat is stored up in the western Pacific Ocean, and then released to the atmosphere in the eastern Pacific. This happens over the course of a few years. El Nino is not predictable, but we can track it in retrospect through sea surface temperature measurements. If we compare past El Nino cycles with temperature changes over the past three decades, we can see that there is a strong relationship between the two. El Nino years tend to be hot years. Recent years have been dominated by the cool phase of the cycle. This is responsible for some of the slowdown in warming. However, El Nino doesn’t explain everything. There are cooler periods in the early eighties and nineties which don’t fit the El Nino cycle.

These were caused by two major volcanic eruptions, El Chichon and Pinatubo. Dust from the volcanoes spread in the upper atmosphere, cooling the surface. Smaller eruptions happen all the time, but can also affect temperatures. There has been an increase in the number of small eruptions over the past few years, offsetting a bit of the greenhouse warming. Another factor is the solar cycle. Satellites tell us that the sun varies in brightness with the sunspot cycle. The last cycle has been particularly weak. A dim sun also offsets a little bit of warming. Yet another factor is pollution. Rapid industrialisation in Asia has led to more particulate pollution in the atmosphere, which also has a cooling effect. The final factor is in the observations themselves. Two of the major temperature data providers, the UK Met Office and NOAA, don’t include the Arctic in their global temperature calculation, because there are no weather stations there.

But the Arctic has been warming faster than anywhere else on the planet. Missing it out leads to an underestimation of the rate of warming. To recap, greenhouse gases have continued to grow over the last one and a half decades. But over the same period, volcanoes, the weak sun and pollution have had a cooling effect, and the rate of warming has been underestimated as well. Two recent studies have put all of these together. If we ignore the short term influences, climate models predict faster warming than we have observed. However, if we use global temperature estimates, and add the influence of El Nino, volcanoes, the weak sun and pollution into the models, then the agreement is good. What can we conclude from this? When we put everything we know into the models, the answers match what we observe. So the slowdown in warming makes sense in retrospect, and doesn’t give us a reason to doubt the models.

However, we couldn’t have predicted it in advance, because we can’t predict volcanoes, pollution or the sun. The slowdown in warming has created a whole family of myths with different levels of sophistication. At one extreme, it is possible to argue that the hiatus should reduce our estimates of climate sensitivity. This is a genuine scientific argument, although the analysis we have just seen suggests that no reduction is required. At the other extreme, it is sometimes claimed that the hiatus disproves the role of CO2 in global warming. They claim that CO2 has increased, but the world hasn’t warmed. This is an example of a strawman, and a complex cause fallacy. Climate science doesn’t claim that CO2 is the only factor which affects temperature. This is why the hiatus is so hard to deal with. The myths may be wrong, but they are simple and convincing. The complex cause fallacy exists because people like things to be simple, but explaining the complex drivers of climate is hard. But in the end, all the hiatus myths revolve around drawing attention away from the big picture. When we look at the big picture, the hiatus does not change our understanding of human caused global warming.