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Global temperature slowdown — not an end to climate change

A decade of little rise in global temperatures

In 1998 the world experienced the warmest year since records began. In the decade since, however, this high point has not been surpassed. Some have seized on this as evidence that global warming has stopped, or even that we have entered a period of ‘global cooling’. This is far from the truth and climate scientists have, in fact, recognised that a temporary slowdown in warming is possible even under increasing levels of greenhouse gas emissions.

The record-breaking temperatures in 1998 occurred after three decades of warming, starting in the 1970s. These decades saw an increase in global average temperature of about 0.45 °C. After 1998, however, warming slowed significantly — trends over the past 10 years show only a 0.07 °C increase in global average temperature. Although this is only a small increase, it indicates that there has been no global cooling over this period. In fact, over the past decade, most years have remained much closer to the record global average temperature reached in 1998 than to temperatures before the 1970s. All the years from 2000 to 2008 have been in the top 14 warmest years on record.

Top 20 warmest years on record
Ranking (1–10) Year Temperature difference Ranking (11–20) Year Temperature difference
1 1998 0.515 °C 11 1995 0.276 °C
2 2005 0.479 °C 12 1999 0.262 °C
3 2003 0.457 °C 13 1990 0.248 °C
4 2002 0.455 °C 14 2000 0.238 °C
5 2004 0.432 °C 15 1991 0.197 °C
6 2006 0.422 °C 16 1983 0.187 °C
7 2007 0.403 °C 17 1987 0.167 °C
8 2001 0.400 °C 18 1994 0.163 °C
9 1997 0.355 °C 19 1988 0.163 °C
10 2008 0.314 °C 20 1981 0.130 °C

 

Temperature difference from long-term average
Ranking (1–10) Year Temperature difference Ranking (11–20) Year Temperature difference
1 1998 0.52 °C 11 2008 0.31 °C
2 2005 0.47 °C 12 1995 0.28 °C
3 2003 0.46 °C 13 1999 0.26 °C
4 2002 0.46 °C 14 1990 0.25 °C
5 2009 0.44 °C 15 2000 0.24 °C
6 2004 0.43 °C 16 1991 0.20 °C
7 2006 0.43 °C 17 1983 0.19 °C
8 2007 0.40 °C 18 1987 0.17 °C
9 2001 0.40 °C 19 1994 0.17 °C
10 1997 0.36 °C 20 1988 0.16 °C

 

Global annual ranked HadCRUT3

Global annual ranked HadCRUT3 graph
Fig 1. Global ranked annual HadCRUT3

The heart of the matter — internal climate variability

After three decades of warming caused by man-made greenhouse gas emissions, why would there suddenly be a period of relative temperature stability — despite more greenhouse gases being emitted than ever before? This is because of what is known as internal climate variability. In the same way that our weather can be warm and sunny one day, cool and wet the next, so our climate naturally varies from year to year, and decade to decade.

Before the twentieth century, when man-made greenhouse gas emissions really took off, there was an underlying stability to global climate. The temperature varied from year to year, or decade to decade, but stayed within a certain range and averaged out to an approximately steady level.

In the twentieth century we have had continued variability, but an underlying trend of warming in the previously steady long-term averages. This is what we observed in the 1970s, 1980s, and 1990s. Now we have seen a decade of little change in the average global temperature — but that doesn’t mean climate change has stopped, it’s just another part of natural variability.

Global temperature differences

Graphic of changes in temperature
Fig 2. Differences in global average near-surface temperatures - 1850 to July 2009.

The ocean

There are a huge number of factors which cause this variability in our climate, but one of the most important is the El Niño phenomenon and its counterpart, La Niña. El Niño years see a shift in Pacific Ocean currents which results in the surface of the ocean heating, creating a warming effect. La Niña brings cooler water to the surface and creates a cooling effect. These processes happen in cycles over many years and, depending on which is in force at the time, can significantly affect global temperatures.

In 1998 El Niño was at a 20th-century peak, which contributed to record global temperatures seen that year. Many climate sceptics point to the fact that 1998 was the warmest year on record, and say that because no year has topped that since, there must have been global cooling. However, to look at one year in isolation is effectively seizing on an extreme of natural variability and using that to judge long-term climate. It’s the underlying trend that is important, which is why you can only make judgements over longer periods of time.

Forecasting our climate

Climate models, which are used to forecast the future of the world’s climate, include most known factors which cause internal variability. Because of this, the projections for global climate do not include continual warming year-on-year. Instead they more closely reflect the reality we would expect, with some years warmer than others and even some series of years cooler than a preceding year.

Recent Met Office research investigated how often decades with a stable or even negative warming trend appeared in computer-modelled climate change simulations.

Jeff Knight, lead author on the research, says: “We found one in every eight decades has near-zero or negative global temperature trends in simulations. Given that we have seen fairly consistent warming since the 1970s, the odds of one in eight suggest the observed slowdown was due to happen.”

Our decadal forecast predicts an end to this period of relative stability after 2010. We project at least half of the years after 2009 will be warmer than the 1998 record. Climate researchers are, therefore, reinforcing the message that the case for tackling global warming remains strong.

Commenting on the new study, Vicky Pope, Head of Climate Change Advice at the Met Office, said: “Decades like 1999–2008 occur quite frequently in our climate change simulations, but the underlying trend of increasing temperature remains. We cannot be complacent. Indeed, other signals of climate change are increasing as fast, or even faster than ever due to the combined effects of global warming and natural variability — the rapid loss of summer Arctic sea ice is one such example. Early action to reduce the extent and impacts of climate change remains vital.”