You could be forgiven for thinking that global warming has paused. It’s a narrative that has been hammered home by columnists and politicians around the world. Proponents of this argument say there is no need for alarm and that fewer resources need to be dedicated to lowering carbon emissions.
But that argument has relied on what has now been proved to be a fault with how scientists measure global temperatures.
The temperature record for the last 18 years has appeared to show that the rate of warming since 1950 has slowed down since 1997. The world’s three major data sets on global temperatures – curated by the Goddard Institute for Space Studies and Surface Temperature Analysis in the United States, the Japan Meteorological Agency and the Met Office Hadley Centre in the United Kingdom – have shown this slowdown.
The United Nation’s climate body – the Intergovernmental Panel on Climate Change – noted in its 2013 report that surface temperatures have shown “a much smaller increasing linear trend over the past 15 years than over the past 30 to 60 years”.
The two arguments are often rehashed in South African media. A 2016 comment piece in the Daily Maverick (“Leo DiCaprio grunts his way to climate sainthood”) talked about “a very clear ‘pause’ in warming”. It also noted: “The longer this slowdown lasts, the less credibility alarmist predictions have.” Similar comment pieces have proliferated in business media.
But two sets of research have removed the factual basis for the argument that global warming has slowed down.
The first came in 2015, from the National Oceanic and Atmospheric Administration in the United States, one of the most important climate observation groups. Its research, Possible Artifacts of Data Biases in the Recent Global Surface Warming Hiatus, found that there has been no pause in global warming.
It concluded: “Our analysis suggests that the apparent hiatus may have been largely the result of limitations in past datasets.”
Instead of a pause, the group said, warming had probably accelerated: “The rate of warming over the first 15 years of this century has, in fact, been as fast or faster than that seen over the last half of the 20th century.”
Their conclusion came from interrogating the data sets being used to show global average temperature. The data for these – published by the three major global climate agencies – comes from a combination of satellite, vessel and ocean buoy measurements.
With a conclusion that ran in opposition to the science of the day, the administration’s work inspired other researchers to start interrogating global weather data sets. One team, working out of Berkeley in California, published their own findings last week in the journal Science Advances. The research, Assessing Recent Warming Using Instrumentally Homogenous Sea Surface Temperature Records, confirmed the 2015 findings of the US oceanic and atmospheric administration.
It focused on the “biases due to changing instrumentation” that make it look like warming has paused. The big three data sets combine information from satellites, vessels and ocean buoys. Records from vessels have been the mainstay of ocean temperature observation – up until the 1940s temperatures were taken by dropping a bucket over the side of a vessel and sticking a thermometer in it.
By 1990, some 80% of all measurements were taken by measuring the temperature of water running through a vessel’s engine room. Given all the variables in this method of recording temperatures, by 2015, 80% of measurements were rather coming from buoys bobbing about the ocean.
But the data sets haven’t made an allowance for this change in the source of data. Instead, what the Berkeley team call “smush” continued – putting all the data into a single temperature for the Earth’s surface. With oceans making up 71% of the surface, any anomalies there are a problem.
The team decided to separate the different sets and looked at their temperatures in isolation. They found that the data coming from buoys tended to record colder temperatures than that from vessels.
By correcting this “cold bias”, they concluded that the oceans have been warming at 0.12°C a decade since 1997. Previous estimates were that the warming had only been of the order of 0.07°C a decade. This correction shows that no pause has happened, and warming since 1997 has been consistent with warming since 1950.
The world is warming. It never stopped warming.