For more than a hundred years, scientists have known greenhouse gases warm the atmosphere. But despite trapping more and more heat, earth’s surface temperature over the past decade and a half has risen slowly. And understanding where the extra heat ends up can be more complicated.
We talk to Dr Richard Allan, lead researcher on a new project called Deep-C, about how the tools to take earth’s temperature have changed – and how new measurements can help scientists investigate what’s behind the surface warming slowdown.
Allan is a climate science researcher at the University of Reading’s department of meteorology. His career has focussed on combining measurements with climate models to understand changes in earth’s climate.
When we talk about the earth’s temperature, we usually mean the temperature of the air above the land and ocean, or surface temperature, as it’s what humans experience most directly. But surface temperature is only a small part of the climate system. In fact, most of the extra heat the planet absorbs goes into the oceans below the surface. Allan tells us:
“The vast ocean has a huge capacity to store heat â?¦ There’s a very good relationship between the extra radiative energy entering the top of Earth’s atmosphere – due to increases in greenhouse gases – and ocean heating.”
Greenhouse gases in the atmosphere absorb and re-emit infrared radiation, causing the climate system to warm. Source: Okanagan university college in Canada.
Some heat also goes into melting earth’s bodies of ice: the Arctic, the ice sheets and mountain glaciers. But compared to the oceans, it’s not much:
“[T]he amount of energy that goes into melting that ice is very small compared to the amount of energy that goes into heating the ocean.”
These heat flows mean that understanding how climate change is affecting the planet means you need to understand more than how surface temperatures are changing, Allan says.
“To balance the books you need to be measuring the energy coming in at the top of the atmosphere and all the places where it’s going to really understand how the climate is heating.”
Improving temperature measurements
Allen tells us that scientists have a wealth of data documenting how air temperatures have changed, dating back from the first thermometers in the 17th century:
“[W]e still use the methods for measuring air temperature over land that we used hundreds of years ago but global coverage increased during the 19th and 20th centuries.”
18th century Swedish astronomer Anders Celsius, who founded the temperature scale which bears his name.
Although the area covered by temperature data has expanded over the centuries, some parts of the world, like the Arctic and Antarctic, still aren’t very well sampled. Data from satellites has helped fill in gaps in temperature measurement, although it can’t take the place of direct measurements just yet. Allan says:
“[W]e’re not really at the stage of accurately measuring temperature over land from satellites only, we still need to use meteorological instruments. With satellites you’re indirectly measuring temperature, so you have to be careful about other factors that can influence what the satellite sees.”
To draw robust conclusions about how the climate is changing, Allan says scientists need data that covers at least 100 years. This means historic thermometer data is still needed to give a full picture:
“[There are] slow natural fluctuations in the ocean … if you’re measuring for a period of ten, 20 or even 30 years, you might just be measuring part of that natural cycle. You need to have a longer term perspective so you can put these bumps and troughs in context of longer timescale changes.”
So is new data telling scientists new things about recent warming? Despite greenhouse gas concentrations continuing to rise at a fairly steady pace, earth’s surface hasn’t actually warmed very much in the last decade and half.
In the past, it’s been difficult to test this theory. But there have been huge advances recently in how scientists monitor ocean temperature.
It used to be that scientists sent instruments over the side of ships to measure temperature to a depth of about 700 m. But that has changed in the past decade with the ARGO project – a network of free-floating buoys traversing the world’s oceans to measure temperatures. Allan explains:
“[ARGO buoys] float freely with the ocean current, sinking down to a depth of 1,800 m then coming back up again to transmit their data to satellites, then sinking back down again, and so on.”
ARGO floats repeatedly sink to a depth of about 1,800 metres then come back up to transmit temperature data via satellite. The lifetime of each float is about four years. Source: Met Office
ARGO floats first started sending temperature data in 2000, but Allan says that coverage has been more or less global since about 2005:
“We now have almost the whole global ocean sampled down to nearly 2,000 metres with these ARGO floats â?¦ There’s going to be some even deeper ARGO buoys in the future which will go down to below 2,000 m, which will help us understand where the heat is going in the oceans.”
Argo floats have been deployed since 2000 and have now near global coverage of the world’s oceans. Source: Met Office
Scientists tackle surface warming slowdown
The Deep-C project brings climate scientists from across the UK together to investigate the root cause of the recent slowdown in surface warming. Allan says the Walker Institute at the University of Reading is collaborating with several other partners on the project:
“We are linking up with NASA’s Langley research centre, who are world leading experts in measuring how much heat is coming into earth’s system. We’re collaborating with the UK National Oceanography Centre who are the leading expertise in the oceans. And we’ve got leading climate modellers down at the Met Office and experts in ocean processes.”
Allan tells us the way climate models and temperature data have matured in the last decade or so means scientists are now well equipped to tackle this fundamental question of recent warming. He says:
“I think we’re very well-placed to be able to tackle the mechanisms of where heat is being [transported] into the ocean â?¦ We now have the tools and expertise available to do this.”
While many seek rapid answers to the question of what’s causing slower surface temperature rise, scientists know it’s likely to be a complicated one to unravel – and one that may take longer than the four-years of the DeepC project.
But Allan is confident the lifetime of DeepC will see key questions starting to be answered and beyond that, drawing together the wealth of data available will be a lasting legacy in terms of how scientists continue to tackle fundamental climate questions in the future.