New paper shows natural climate cycles can change the pace of atmospheric warming
Despite greenhouse gas concentrations rising, one part of the climate system – earth’s atmosphere – has warmed more slowly over the last decade or so than in previous years. Scientists have been speculating why and a new paper suggests it could be down to a natural climate fluctuation sending heat into the deep ocean rather than the atmosphere. We take a closer look.
Scientists know greenhouse gases are driving up global temperature. But data on land and from the surface of the ocean in the last decade and a half show surface temperatures have risen somewhat slower than expected.
And as we reported recently, the current period of slower temperature rise was preceded by period of faster than expected warming around the year 2000, lasting several years.
So how unusual is such variation? According to a paper just published in the Journal of Climate, this isn’t a new phenomenon. Some decades in history have shown a rapid rise in global temperature – or accelerated warming – while in others there’s been very little or no warming trend – which the paper calls “hiatus” decades.
The research suggests that we’re in one of these so called hiatus periods at the moment. But what’s causing it? According to the new paper, understanding natural variability in the climate system could be key to finding out.
Oceans are important
When looking at how global temperatures have changed, it’s easy to focus on the atmosphere because that’s where most measurements are made. But the heat that stays trapped in the atmosphere is only a small fraction of the sun’s energy that hits earth.
A recent analysis of temperature measurements throughout the global ocean showed that at the same time atmospheric temperature rise has slowed over the past decade or so, the rate at which the oceans take up heat has accelerated. We reported on the paper here.
The oceans are capturing more heat than they used to, but ocean warming isn’t evenly distributed. While the deeper ocean has experienced accelerated warming since 2000 at a rate unprecedented in the last 50 years, in the top 300 m of the ocean – shown in grey in the graph above – warming has slowed slightly since 2000.
Natural fluctuation
The new ‘hiatus’ paper suggests a reason why the oceans could suddenly start to take up more heat like this. It’s a natural cycle in the ocean known as the Interdecadal Pacific Oscillation (IPO).
Over timescales of a decade or so, the IPO flips between a positive phase – which tends to warm the earth’s surface – and a negative phase – which has a cooling effect.
The researchers found that over the 20th century, decades with faster warming are generally associated with the positive phase of the IPO. During these periods, manmade warming combined with a positive IPO leads to faster warming of surface air and surface ocean temperatures, with little heating of the deep ocean.
The opposite occurs when the IPO is in the negative phase. During those periods, the cooling effect the IPO has on global surface temperatures slows manmade warming – leading to a slowdown in atmospheric and surface ocean warming, but rapid deep ocean warming. And that’s the situation we’re seeing now, the paper suggests.
Past form
Scientists have linked hiatus periods to particular phases of the IPO before. A previous study showed the positive phase of the IPO contributed to a period of faster warming in the 1970s.
In the late 1990s, surface warming slowed for a decade or so – which scientists have also attributed to the transition of the IPO from its positive to negative phase.
Other players
But the IPO isn’t the only factor that could be contributing to the increase in deep ocean warming during hiatus decades. The authors found elements of a major ocean circulation pattern called the Thermohaline Circulation have some effect on ocean warming during positive phases of the IPO.
But nevertheless, the study found the IPO has the most consistent contribution to faster and slower warming decades in the 20th century.
Future hiatuses
The study suggests two things about the current “hiatus” period in global surface temperature. As co-author Professor Kevin Trenberth told Carbon Brief recently, the first is that it is a temporary phase that should not be taken as evidence that global warming has stopped or even slowed. He says:
“[The findings] means that the current hiatus in surface warming is a transient and global warming has not gone away.”
What’s more, when the authors looked at climate model projections of the future under one of the new medium emissions scenarios developed by the Intergovernmental Panel on Climate Change (IPCC), they found sequences of faster and slower decades over the next century related to when the IPO is expected to change phase. So we should expect more hiatus periods in the coming century.
The hiatus periods in the model simulations last up to 15 years, which the authors suggest means the current one could last another few years.
It’s clear the oceans have a major role to play when trying to understand how sensitive the atmosphere is to increasing greenhouse gases – a concept known as the climate sensitivity. A number of lines of evidence now point to how the earth continues to warm, even in periods where atmospheric temperatures are rising more slowly than in the recent past.