Finding the fingerprint of human influence on climate

Scientists have spotted a historical pattern of human activity on global droughts. The study shows how the effects of global warming from fossil fuel use have been influencing droughts since the beginning of the 20th century.

A thorny issue that often comes up in climate science is whether specific weather events such as a drought or hurricane are the result of climate change or the products of natural variations the Earth goes through. After all, there are many natural cycles of warm and cold. 

Some work over years or decades, such as El Nino and La Nina, or the Pacific Decadal Oscillation. Others take thousands or millions of years, such as ice ages or Melankovitch Cycles that relate to changes in the orbit of the Earth around the sun.

On top of that, there are other irregular events such as volcanic eruptions that can temporarily cool the climate. With so many natural cycles overlapping with each other, it is sometimes difficult to tease out the added effect of human carbon emissions on the atmosphere.

Now, researchers at NASA's Goddard Space Flight Center have put together a global "drought atlas" reaching back more than a century. They found a global pattern of droughts that fit with human activity since the beginning of the Industrial Revolution.

This study goes way beyond attributing individual droughts or hurricane events to climate change. It looks at the global, historical pattern — or "fingerprint" — of droughts which is likely to grow stronger over the next few decades, according to this new research.

Tree rings hold the key for this historical perspective

The key to spotting the fingerprint was gathering drought data from around the world from several different sources. Since the 1980s, satellites have been monitoring soil moisture around the globe, an indicator of wetness of drought. To fill in the historical data, the scientists looked at ground reports on soil moisture and tree rings.

Trees hold a precise seasonal record of rainfall in the width of their annual rings, which grow thicker in wet periods and thinner during drought. Using tree ring data from around the world, they provided a detailed climate record reaching back a thousand years.

The human fingerprint on climate showed up around 1900 and continued into the first half of the 20th century. During this time, periods of drought and wetness were not just triggered by local conditions, but rather happened simultaneously in different parts of the world, suggesting they were influenced by a larger system of global climate change. The beginning of the 20th century is right after the start of the Industrial Revolution when burning fossil fuels began in earnest.

This pattern of human influence on droughts faded in mid-century, between 1950-75, believed to be caused by unchecked pollution from all the vehicle tailpipes and industrial smoke stacks that put aerosols into the atmosphere and smothered cities in smog.

These artificial clouds affected aspects of regional precipitation, blocked sunlight and slowed the rate of climate warming. But since air pollution laws have been put in place that require catalytic converters on cars and restricted emissions from industry, the air has become clearer, and now there is evidence that the fingerprint on droughts is returning.  

As more areas of the world face water shortages, longer fire seasons and crop failures, the effects of climate change are becoming more visible — and economically costly. The lesson from this report is that these effects of greenhouse gas emissions on the climate are not just showing up now, they've been with us for a long time.