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Climate change has been implicated in record-breaking temperatures across the 20th century. KayVee.INC/Flickr, CC BY-NC

We traced the human fingerprint on record-breaking temperatures back to the 1930s

In recent years climate scientists have looked at the role climate change played in unusual extreme weather events such as Australia’s hottest summer in 2012-13 and recent heatwaves.

Before now no one had looked at how far back in time we could go and still link these weird weather events and record-breaking climate extremes to our influence on the climate.

Our study, published in Geophysical Research Letters, addressed the question of when climate change started altering the influence of record hot years and summers in a way we can detect. We looked at five regions of the world, as well as the whole globe.

We’ve been changing the weather for a long time

Human-made climate change has been influencing heat extremes for decades, with many past records directly attributable to the effect we have had on the climate.

Number of record-breaking hot years and summers attributable to the human influence on the climate Author provided, Author provided

The map above shows how many record-breaking hot years and summers we can attribute to climate change in different parts of the world.

We found the last 16 record-breaking hot years globally up to 2014 were made more likely because of climate change (we didn’t include 2015 – the current holder of the hottest year record – because we performed our analysis before the end of last year). The earliest year we found that humans had contributed to temperature records was 1937. Since then every record-breaking year globally (all 15 following 1937 that were the hottest up to that time) can be attributed to climate change.

The record-breaking hot years the world has experienced in the last couple of decades would have been almost impossible in a world without climate change.

Even on a regional scale we see many record-breaking hot years and summers where the fingerprint of climate change is clear. The last six record hot years and three record hot summers in Australia were made more likely by the human influence on the climate.

Hot and cold

In other regions of the world, human activities have also had a cooling effect through increasing aerosols in the atmosphere (due to industrial activity and the emissions of particulates from cars and airplanes, for example). Here fewer record heat events show as clear a human influence. But even in areas such as eastern Asia the overall human influence on the climate has increased the likelihood of hot years and summers back to the 1980s.

Central England has had no record-breaking hot summers since the extreme heat of 1976. Any future hot summer that breaks that record will be associated with climate change.

In fact, for all the regions of the world we studied, the influence of climate change on record-breaking heat extremes has been increasing through time. For instance, we find that the Australian record hot year of 2013 was 22 times more likely due to climate change, whereas if we look back at the 1980 record we find only a three-fold increase in likelihood of that record from human influences (see here for an explanation of how these studies are done).

As we have exerted a greater influence on the climate, by increasing the concentration of greenhouse gases in the atmosphere, the likelihood of record-breaking heat extremes has increased.

This study shows that the human influence on record-breaking hot extremes extends back many decades, especially when we look at the globe and Australia. While our aerosol emissions have delayed a clear human influence appearing in other areas of the world, the fingerprint of climate change has become clear over recent decades as the warming influence of greenhouse gases has overtaken the cooling influence of aerosols.

Reduce emissions and we should get fewer records

Other factors also influence the likelihood of record heat events occurring – for example, El Niño events. We looked at only the human influence in this study. Undoubtedly natural variability was important in many of these events occurring; by isolating the human influence we found that climate change played a major role in many record heat events as well.

Any hot years or summers in the near future in these regions will be strongly linked with climate change. This analysis was conducted before the end of last year, so although we didn’t include it we would expect a similar result for the 2015 global heat record. Already, 2016 is expected to challenge that record.

Whether we continue to experience such frequent record-breaking heat extremes will in part depend on whether we reduce the influence on the climate as the Paris agreement sets out to do.

If we reduce our emissions, we may not see as many future record hot summers and years. The impacts these events have on society and the environment may be reduced.

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