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Evidence of man-made climate change identified starting as early as 1900

Evidence of man-made climate change has been found dating back to 1900, according to a new study from the Nasa Goddard Institute for Space Studies.

Global warming fingerprints can be seen in patterns of drought and rainfall across the world from that time and the pattern exacerbates as emissions increase over time.

The team said the study is the first to provide historical evidence connecting human-generated emissions and drought at near-global scales, lending credibility to forward-looking models that predict such a connection.

The study’s key drought indicator was the Palmer Drought Severity Index, or PDSI. The PDSI averages soil moisture over the summer months using data such as precipitation, air temperature and runoff.

While today Nasa measures soil moisture from space, these measurements only date back to 1980. The PDSI provides researchers with average soil moisture over long periods of time, making it especially useful for research on climate change in the past.

The team also used maps of where and when droughts happened throughout history, calculated from tree rings. Tree rings’ thickness indicates wet and dry years across their lifespan, providing an ancient record to supplement written and recorded data.

Among the observations, researchers documented drying of soils across much of North and Central America, Eurasia and the Mediterranean.

Other regions, including the Indian subcontinent, had become wetter over the past century.

The study suggests that changes linked to global warming caused by human activity may have been going on longer than previously thought.

Study co-author Dr. Benjamin Cook, from the Nasa Goddard Institute for Space Studies and Columbia University, said: “We asked, does the real world look like what the models tell us to expect?

“The answer is, yes. The big thing we learned is that climate change started affecting global patterns of drought in the early 20th century.

“We expect this pattern to keep emerging as climate change continues.”

Colleague Dr. Kate Marvel, a climate modeller at Goddard and Columbia, said, “It’s mind boggling. There is a really clear signal of the effects of human greenhouse gases on the hydroclimate.”

The global warming fingerprint was most obvious between 1900 and 1949, said the scientists writing in the journal Nature.

From 1950 to 1975, the pattern scattered into seemingly random events - possibly due to enormous amounts of industrial aerosols pouring into the atmosphere without pollution controls.

The effect may have been to mask the impact of greenhouse gases, even though their levels continued to rise, said the scientists.

Then starting in the 1970s, many industrial countries began to introduce progressively stricter clean-air laws. At the same time, greenhouse gas emissions spiralled upwards.

As a result, the global warming signature became more visible again in the early 1980s.

The signal is still in the process of strengthening, the researchers point out.

Dr Marvel said: “If we don’t see it coming in stronger in, say, the next 10 years, we might have to wonder whether we are right. But all the models are projecting that you should see unprecedented drying soon, in a lot of places.”

Many centres of agricultural production could become permanently arid, especially in North America and Eurasia, the scientists warned.

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