Summers feel hotter on a warming planet
If you feel like summers have been getting warmer, new research suggests you might be onto something. A new study shows that in many parts of the world, summers have got significantly warmer in recent decades, in some areas since as early as the 1960s.
In the study, scientists looked at the speed at which local temperatures have changed across the globe since 1920. They found that in many locations, summer temperatures have gone up by a statistically significant amount due to manmade climate change. Many parts of the world are now experiencing a new summer climate, say the researchers – it’s not only on the global scale that temperatures are changing.
The research, carried out by a team of researchers from the US National Oceanic and Atmospheric Administration and the University of Colorado at Boulder, is published in Geophysical Research Letters.
Temperature trends
Temperature measurements from around the world tell us that the earth has warmed by about 0.8 degrees since 1900. This trend is clear because the change in average temperature exceeds the bounds of natural variability – and persists for more than a few isolated years.
The new study shows the same kinds of trends can be found on a smaller scale. The researchers analysed monthly temperature measurements from different regions of the world, collected as part of the Met Office and the University of East Anglia’s CRUTEM4 data set. The researchers looked at 90 years of data from 1920 to 2010. They compared temperatures using 30-year windows over that time frame, running the comparison every 10 years.
The researchers calculated whether the range of temperatures in each test period was statistically different from the last. When they found a significant difference that continued into following decades, they concluded that a trend had emerged.
Warming summers
The results showed that in many low latitude regions, including southern Europe, India, southern north America, South Africa and parts of Australia, summer temperatures have already increased, in places by up to 1.5 degrees Celsius. In some cases, this has been the case since the 1960s. The change in temperature seen exceeds that which can be expected from natural variability in the climate.
Image - Trend _emerging (note)
This map shows the difference in mean summer temperature between the base period (1920 and 1949) and when a trend is detected. Most of the locations show a warming signal, up to 1.5 degrees Celsius. Source: Mahlstein et al., (2012).
The data showed a trend towards warmer summers in 40 per cent of tropical areas (north and south of 22.5 degrees latitude) but only 20 per cent outside the tropics.
The researchers say that in higher latitudes, including northern Europe and northern North America, a clear trend in temperature change can take longer to emerge because in these regions there’s a bigger natural variation in temperatures. In lower latitudes, temperatures vary less widely so changes are easier to spot.
So even though temperatures have risen more in higher latitudes, these changes are harder to isolate, as lead author Irina Mahlstein explains in the paper:
“Despite the fact that northern latitudes show larger warming signals, the signal is still not large enough to emerge from the variability”.
Ecosystem impacts
Living creatures are built to cope with changes in their environmental conditions within the range of what might occur due to natural fluctuations in the climate system – known as natural variability.
In places where the scientists detected a temperature rise outside of what could be expected naturally, temperature change could already be adversely affecting individual species and crop yields. Mahlstein continues:
“When a local temperature regime is moving away from the known climate, thus showing an emerging signal, ecosystems are likely stressed by these changes.”
Before now, some climate models have been able to detect trends in regional as well as global temperatures. This study is the first to use only observations of temperature – not computer simulations – to show that some regions are experiencing a new summer climate regime, a change that is detectable over the period of a single lifetime.
The scientists say that it’s unlikely the same trend could be found in rainfall, because year-to-year variability is much greater.
Not all locations show a trend towards warmer summers. A few locations in northern high latitudes showed cooling between the late 1940s and the 1990s. The scientists attribute this to a cooling following strong warming of the early 20th century, which was particularly pronounced in regions around the North Atlantic.
These findings highlight that although global average temperature is an important measure of the climate, what’s happening to local temperatures has more relevance when it comes to looking at the effects of climate change, and it’s beginning to be possible to see changes in local temperatures – even over a person’s own lifetime.