Global warming slowdown? The argument is as clear as an unseasonal blizzard

The debate surrounding whether climate change is slowing down or not is certainly speeding up, and it's becoming far more complex than anyone could have predicted.

A flooded street in Dubai last month where strong wind caused a crane to collapse. Antonie Robertson /The National

The recent run of chilly rain and snow in the UAE seems to affirm the UN’s admission in 2013 of a decline in temperature rises. But the complexities of time and maths make it hard for scientists to say whether that is permanent.

Snowball fights in the UAE, snowless slopes in the Alps. Chilly winds in Dubai, balmy weather in Minnesota.

Another winter, another outbreak of weird weather. Still, that is climate change for you.

Or is it? Ask a local taxi driver and you may end up in a debate about how plunging temperatures can be squared with global warming.

So what do the experts say? At last month’s regional meeting of the World Meteorological Organisation in Abu Dhabi, the talk was of how warming Arctic air and declining sea ice is affecting the flows of air and seawater that influence the region’s weather.

In particular, the polar jetstream – the band of fast-moving air that can stop polar air from reaching farther south – has been flailing around like a snake in a sack, flipping the weather around in a heartbeat.

But do not expect climate experts to seize on the recent bout of freak weather and insist it must be man-made global warming. They know it is all more complex than that.

From the strange, barely predictable temperature changes in the Pacific, known as El Nino, to random upheaval, global warming is not the only influence on the weather.

And according to some, it may no longer be the threat it once was.

The idea that global warming may be grinding to a halt has been around for a decade, and is based on data collected from thousands of weather stations around the world. When plotted against time, the temperature measurements produce a zig-zag pattern, with some years cooler and others warmer than before. The long-term direction is clear enough, however: upwards.

But around 2007, some researchers began pointing out that the trend seemed to be breaking down.

Initially, many dismissed the claim as simply part of a denialist agenda to discredit the concept of global warming. Yet, as the years rolled by and more data came in, it became harder to dismiss.

In 2013, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) agreed that a slowdown was under way. The data pointed to a warming rate from 1998 onwards that is barely half that of the previous half-century’s.

And for reasons unknown, the slowdown had not been predicted by computer models of the climate.

Unsurprisingly, climate-change sceptics seized on the IPCC’s “admission” as proof that the models could not be trusted to predict global warming.

Some scientists suspected, however, that the problem might lie elsewhere – namely, with the raw data.

In 2015, a team led by Thomas Karl, at the US National Oceanic and Atmospheric Administration (NOAA), pointed to changes in temperature measurement techniques that could have introduced subtle bias into the data.

Sure enough, once these were corrected, and new data from more sites added in, the slowdown vanished – suggesting the models were correct after all.

Although hardly a ringing endorsement of the reliability of global warming data, the NOAA’s findings were welcomed by many climatologists. Hard science, it seemed, had once again defeated the deniers.

But the story did not end there. Last year, the Nature Climate Change journal published work by another team that claimed the newly-corrected data were still biased – this time by subtle atmospheric influences on the Earth's temperature.

When these were taken into account the slowdown appeared again, although less strongly and over different timescales.

Last month, the story took a more dramatic turn. A former data scientist at NOAA alleged the Karl paper had been rushed out without proper checks.

Whether the allegations are really all that serious remains unclear.

Sceptics view them as proof of the questionable nature of much climate research. Many climatologists dismiss them as nitpicking. One commentator even declared the whole controversy to be “fake news”.

Although that may be pushing it too far, the continuing debate does highlight the limitations of science as a means of checking “alternative facts”.

Those involved in research know that the scientific process is shockingly simple to subvert – inadvertently or otherwise.

For on the face of it, what could be simpler than telling whether something is getting hotter or not?

If your exposure to science stopped at school, you would know exactly what to do: stick a thermometer on it, measure the temperature over time and see if the resulting graph rises, falls or stays the same.

Telling if the entire Earth is getting warmer is a different ball game, however. Simply collecting readings from weather stations is not going to be enough: the data will be plentiful near towns and cities, far less so in remote areas – and virtually non-existent over much of the oceans, which cover most of the Earth’s surface.

Measuring the rate of warming raises another, tougher problem: what is the relevant timescale?

A few years are not long enough: the Earth’s climate is affected by a host of influences that ebb and flow from months to millennia.

Several decades of data are probably the minimum needed to reveal a genuine shift.

Certainly, claiming a hiatus in global warming on the basis of data from a handful of years is premature.

Finally, there is the problem of deciding if any detected trend is real or just a fluke.

The textbook way of deciding is to use so-called significance tests, statistical methods that show the chance of getting the observed results, assuming they are a fluke.

Yet statisticians have been warning researchers for decades that these techniques are prone to mistaking random noise for genuine effect.

They have come under renewed scrutiny recently because of their role in the so-called “replication crisis” in research, in which many highly cited advances fade away on reinvestigation.

Last year, the American Statistical Association took the unprecedented step of issuing a public warning about the dangers of using significance tests.

The argument over the strength of global warming shows how difficult climate research really is.

It also shows the naivety of thinking that on “hot button” issues such as global warming, science can debunk fake news in a flash.

Robert Matthews is visiting professor of science at Aston University, Birmingham, UK