Droughts and heatwaves cause 10% drop in annual crop harvests

Over the past five decades, hundreds of droughts and heatwaves have struck countries across the world. A new study finds that these events caused an average annual drop in national crop production of around 10%.

The cumulative global losses of these hot and dry extremes amount to 3bn tonnes of cereal crops, the study says – equivalent to three times the global maize harvest last year.

Substantial damage

The new study, just published in Nature, estimates average national cereal production losses during each extreme weather disaster occurring between 1964 and 2007.

The findings suggest that heatwaves and drought events substantially damage crop production across the globe.

You can see their results in the infographic below. The main bar chart shows that, on average, crop production drops by 10% in a drought (red bar) and 9% during a heatwave (orange bar) compared to the three years before and after the event.

The impact was generally confined to the duration of the drought or heatwave, the researchers say, with crop production recovering to normal levels the following year.

Main chart shows the change in crop production during droughts (red bars) and heatwaves (orange bars), as a percentage difference from normal years before and after the event. Smaller chart shows average impact of droughts by region. Credit: Lesk et al. (2016).

Bigger losses in rich countries

As you can also see from the smaller chart in the infographic, the researchers found that droughts caused bigger percentage losses in crops (in terms of weight) in richer countries of Europe and North America (purple bar) than poorer countries in Africa (green) and Latin America (blue).

For example, the technically developed farming systems of North America, Europe and Australasia experienced an average drop in crop production during a drought of 19.9%, while in African countries, losses averaged 9.2%.

While this might be a surprise, the findings do make sense, says Prof Andy Challinor, professor of climate impacts at the University of Leeds, who wasn’t involved in the study. He explains to Carbon Brief:

On the whole, crops in developing countries are not optimised for existing climates, but are instead bred to be hardy and produce modest yields with limited agricultural inputs and variable rainfall.

In western countries, farmers tend to have better access to fertilisers, pesticides and crop insurance, allowing them to focus on single crop varieties to maximise yields. This approach, known as monoculture, can leave harvests susceptible to extreme weather, says co-author Prof Navin Ramankutty, professor in global food security and sustainability at the University of British Columbia. He tells Carbon Brief:

Monoculture farming systems can be less resilient to weather shocks compared to a more diverse farming system that tends not to put all its eggs in one basket.

But it’s important to note the results don’t suggest that richer countries are more vulnerable to extreme events, points out Challinor:

It is clear that developing countries are the most vulnerable since they cannot simply pay higher prices for food and because their yields are far lower to start with.

Increasing drought losses

Focusing on droughts specifically, the researchers found the impact on crop production was greater for more recent events. Dividing the data between 1964-84 and 1985-2007, average drought losses were 6.7% during the earlier period and 13.7% in the latter.

This could be down to combination of factors, the paper says, such as increasing vulnerability of crops, improvements in data collection and, potentially, an increase in severity of and exposure to droughts.

The finding suggests that risks to cereal crops could intensify as the climate changes, says Ramankutty:

Climate models project that extreme events will increase in frequency and/or severity in the future. This is especially the case with extreme heat. So we anticipate that a more volatile future climate will lead to more crop production losses from extreme weather disasters.

For western countries, dealing with this risk requires action, warns Challinor:

We can expect crop yields to become increasingly subject to shocks as climates continues to change. The study presents another timely warning for developed countries not to remain locked in to current food production systems, since they tend to be optimised for recent climates.

The UK Global Food Security programme looked at the threat to global food supplies from extreme weather last year. You can read more in their report.

Weather-related disasters

To generate their results, the researchers analysed around 2,8000 weather-related disasters using data from the Emergency Events Database (EM-DAT), maintained by the Centre for Research on the Epidemiology of Disasters (CRED) in Belgium.

They cross-referenced the disaster data with country-level crop production statistics for 16 cereals, such as maize, wheat and rice. Ramankutty explains to Carbon Brief:

[Our approach] asks whether there is a signal in production data that corresponds to when the disasters occur. To do this we have to separate out the signal (production loss from disaster) from the noise (production goes up and down each year due to multiple reasons), which our statistical method helps us do.

The researchers measured damage to crop production in two ways: declines in crop yield and a reduction in how much land the farmers harvest. The latter is often overlooked, but is an important aspect of how extreme events affect cereal production, says Ramankutty:

[It decreases] when the entire crop fails to such an extent that the farmers decide not to harvest the crop at all. It is not worth it to the farmer to spend the effort to harvest.

The researchers also considered floods and extreme cold events in their analysis, but didn’t find a significant impact on crop production.

This could be because floods and cold weather extremes tend to occur during winter and spring – particularly for temperate regions of the northern hemisphere. This means they strike outside of the main growing season and so affect crop yields less, the paper suggests.

Floods also tend to be more localised than heatwaves and droughts, the study adds. This means that their impact may not have such an effect on the national-level crop production data the study uses.

Main image: A farmer walks in a field on the outskirts of Luoyang, central China’s Henan province. © Carlf Zhang/Reuters/Corbis.