What was the ‘hiatus’ all about?

From 1998 to 2012, the average temperature at Earth’s surface appeared to rise more slowly than in the previous few decades. This gave rise to the incorrect claim that global warming had stopped.

Scientists are now confident that any apparent ‘slowdown’ was caused by natural factors temporarily offsetting the warming trend caused by human activities.

Sea level rise, glacier melt and ocean warming continued during this period, so any change in surface warming does not in any way undermine global warming theory. What’s more, any apparent slowdown has come to an end, with the past five years (2014-2018) being the hottest five-year period on record.

Measuring global temperature

Measuring the average temperature of the Earth’s surface and building a record of how it has changed is not a straightforward matter. To get a complete picture of the whole planet, scientists use measurements from thermometers in weather stations on land, research stations, ships and buoys around the world and satellites.

However, weather stations are scarce in some regions, such as Africa and the Arctic, and satellites also miss out some parts of the planet. Historical measurements were not always accurate, and changes in the methods used to measure temperatures can complicate comparisons across time.

Ocean buoy
An ocean buoy measuring air temperature at the sea surface. Image: Wikipedia

What the science tells us

Since records began in 1850, there has been a very clear long-term trend of rising global temperature. But it hasn’t been a steady pace of change. In 2013, the Intergovernmental Panel on Climate Change (IPCC) concluded that from 1998-2012, warming at Earth’s surface occurred at rate of 0.05ºC per decade, a fair bit slower than the average between 1951 and 2012 of 0.12ºC per decade.

So what does this mean?

Periods of slower and faster warming are not unusual in Earth’s temperature record. For example, natural climate variability and more aerosols released as a result of rapid industrialisation following World War II led to a period of cooling in the 1950s and 1960s.

Nevertheless, what caused the apparent slower surface warming from 1998-2012, despite greenhouse gas concentrations rising, has been an interesting question for scientists. The 2013 IPCC report attributed it to a combination of natural fluctuations in Earth’s climate and a small change in the amount of volcanic and solar activity. But since then, scientists’ understanding of what happened has got much better.

First, trends over short time periods are very sensitive to their beginning and end dates because of natural fluctuations in Earth’s climate that temporarily counteract or enhance the long term warming trend. The starting date of 1998 coincided with a very strong El Niño event, which made 1998 an exceptionally warm year. So a trend that begins in 1998 will naturally appear to show less warming than one beginning either side. For example, taking 1996 as the starting point instead suggests a warming rate of 0.14 ºC per decade, which is faster than the long term average.

Starting date aside, scientists now know that a greater proportion of the extra heat trapped in the Earth system by greenhouse gases went into the deep ocean during the apparent ‘slowdown’ period, with a smaller proportion staying in the atmosphere. The ocean needs only a slight increase in ocean heat uptake to slow atmospheric warming markedly.

Other research has found that climate models may have overestimated solar radiation and underestimated the cooling influence of volcanic activity at the start of the 21st century, which helps explain why they couldn’t reproduce the slower warming rate that was apparent in the observational data.

Does the record really show a slowdown?

Other studies looking at the global temperature record itself have also helped to shed light on the period of slower surface warming.

One major such study, conducted by the US National Oceanic and Atmospheric Administration (NOAA), suggested that once the global surface temperature record was updated to include missing data from the fast-warming Arctic and more comprehensive ocean surface temperatures, the apparent drop warming rate over 1998-2012 effectively disappeared. This gave rise to the argument that the slowdown had only ever been an artefact of imperfect temperature measurements.

A study in 2016, however, suggested that even with the updated temperature data, a ‘slowdown’ in surface warming still exists if the aerosol-induced cooling of the 1950s and 1960s is excluded from the reference period. Scientists have published a number of studies and commentaries since then discussing the rate of recent warming, with most of the argument about whether a slowdown existed or not coming down to statistical semantics.

Temperature anomaly for October 2015
Temperature anomaly (difference) for October 2015 compared with 1951-1980 baseline. Image: NASA GISS

Has the ‘slowdown’ story ended?

Statistics aside, it is clear from the years since that warming has not stopped. The last five years (2014-2018) has been the hottest five-year period on record. Eighteen of the 19 hottest years on record have all occurred since 2001 (the strong El Nino in 1998 is the exception). Scientists’ latest estimate is that humans have caused approximately 1ºC of global warming since pre-industrial times.

Scientists’ understanding of the physical processes that can temporarily push the rate of surface warming up or down has come on leaps and bounds because of interrogating the apparent slowdown question. But perhaps the most important lesson is that knowing how humans are affecting the climate means more than just what’s happening at Earth’s surface.

As long as greenhouse gas concentrations keep rising, the ocean will continue to warm, sea levels will continue to rise, ice will continue to melt, and species will continue to migrate away from the Equator towards the poles.