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The low-down on the slowdown

Scientists from the Met Office recently presented the latest data on global temperatures to the UK’s media. We caught up with Professor Stephen Belcher, the Met Office Chief Scientist, to talk about the end of the global warming slowdown.

Q. Much media attention was given to the idea that global temperatures stopped rising, the so-called ‘slowdown’. What does the latest data show us?

A. From 1999 to 2014 Met Office figures showed that the rise in the average global temperature had slowed compared to trends in the 1990s. However, latest data from the Met Office show that this slowdown has come to an end.

Q. So have we seen a large change in recent temperatures?

A. Global temperature values in 2015 and 2016 both broke records and were about 1 °C higher than pre-industrial levels, which is taken to be the period between 1850 to 1900. So the data shows that the long-term rate of global warming has now returned to the same level reported during the second half of the 20th century.

Q. And does it look like 2017 is going to be a record-breaking year too?

A. We don’t think it will be. So far 2017 has already exceeded the 1 °C threshold, but the temperatures this year aren’t influenced by El Niño conditions, which tends to increase the global mean temperature. So this leads us to think that it won’t be a record-breaker.

Q. What do you think is behind the rise?

A. Greenhouse gases and aerosols explain much of the long-term increase in global mean temperature, but natural variations in climate also have an influence on the warming over shorter periods. For example, an event such as El Niño tends to lead to a higher global mean temperature. That can go the other way too. Variations in climate in the Pacific led to the slowdown that we saw from 1999 to 2014.

Q. In what way did the Pacific influence these recent figures?

A. The surface temperatures of the Pacific Ocean can oscillate between warm and cool phases. It’s called the Pacific Decadal Oscillation, and until fairly recently the oscillation was in a negative phase. That meant the ocean was subducting heat into the deep ocean, and reducing the warming at the surface, which is what is measured by the global mean temperature. Now the Pacific Decadal Oscillation has entered its positive phase, and we’re seeing warming of the tropics, the west coast of North America and the globe as a whole.

Q. Is this warm phase here to stay?

A. No one can answer this question! We would need to understand the Pacific Decadal Oscillation in much more detail before we could make any predictions about how long we will stay in this warm phase.

Q. Don’t we see slowdowns of the temperature rise fairly often?

A. We saw a slowdown in the early 20th century, and a slowdown between the 1950s and the 1980s. For the first slowdown, temperature records show there was a lot of ‘noise’, which is what might be expected by natural variability. In other words, this is what happens in a climate with little human influence.

For the second slowdown, there is a strong effect from aerosols, which are small particles in the atmosphere natural sources and human activities. The high levels from human activities during the second slowdown led to what is known as ‘global dimming’. Clean air legislation reduced emissions of these aerosols, and we’ve since seen global brightening. In the absence of aerosols masking global warming, the warming of our climate by greenhouse gases is now more apparent.

Q. So should we expect more slowdowns in the future?

A. Well, natural influences like the Pacific Decadal Oscillation and El Niño will continue to affect global temperatures and it is difficult to predict exactly when these will happen. But, overall, an increase in greenhouse gases in the future will mean that global temperatures will continue to rise, even if this rise isn’t perfectly smooth.

Eye on the storms

The year 2017 is bound to go down in the history books for the severity of the hurricane season. Hurricane Irma in particular caused devastation across the Caribbean, wrecking infrastructure and causing fatalities on islands like Barbuda.

The year saw a heavy Southern Asia monsoon too. So can we say these events are a direct cause of climate change? Professor Belcher is quick to urge caution: “The only way to properly determine that would be by comparing weather events with what would have been the case without climate change,” he points out. “However, we do already know that a warmer atmosphere can hold more moisture, which can lead to increased rainfall.”

Another factor that may have an influence is rising sea levels caused by climate change. Any hurricane storm could have more impact because of rising seas, creating an additional peak to any surge.