Q&A: Are the 2019-20 locust swarms linked to climate change?

§ Is East Africa’s current locust outbreak unprecedented?

For months, locust plagues stretching tens of kilometres in length and breadth have blighted central and eastern Africa, the Middle East and parts of Asia, including Pakistan and India.

The swarms first arrived in the Horn of Africa at the end of summer 2019. They had moved in from the Arabian desert, where good breeding conditions in the months prior had allowed them to multiply by an astonishing 8,000-fold, according to the UN’s Food and Agricultural Organization (FAO).

In the autumn months of 2019, the insects spread further inland from Somalia and Ethiopia into countries including Kenya and Uganda. In Kenya, the FAO’s locust watch service identified one swarm that was up to 60km long and 40km wide – roughly the size of Luxembourg. The swarm was made up of up to 20bn individual locusts, officials said.

At the same time, locusts moved from the Arabian desert into Middle Eastern countries, such as Iran, Bahrain and Kuwait, and into parts of Asia, such as India and Pakistan.

By January and February of this year, the locusts had moved into Eritrea and threatened to spread further to Sudan, parts of the Democratic Republic of the Congo and Tanzania. China has also begun to brace itself for a possible outbreak.

They have devoured hundreds of kilometres of crops across East Africa, including in the Rift Valley. This could have particularly “severe consequences”, the FAO says, given the region is home to almost 12 million food-insecure people.

In Ethiopia, the swarms also caused a plane to veer off course after insects slammed into its nose, engine and windshields.

In its latest update on 2 March, the FAO’s locust watch service said the threat from locusts currently remains high in East Africa, parts of the Middle East and Pakistan.

It also warned of large increases in locust numbers in the coming months. Parts of East Africa including Kenya and Somalia could see 400-fold increases in swarm sizes by June as the insects reproduce once again.

The map below, provided by the FAO locust service, shows where insect numbers are likely to increase between March and June.

(UPDATE: On 14 April, the FAO Locust Watch released a further statement saying that “widespread rains…in late March are expected to cause a dramatic increase in locust numbers in East Africa, eastern Yemen and southern Iran in the coming months”. Efforts to control locust numbers have been hampered by the coronavirus outbreak, which is causing delays to pesticide deliveries across country borders, Kenyan officials told the Guardian.)

(UPDATE: The locust swarms moved further into western India from Pakistan and Iran around mid-May, according to an FAO Locust Watch update. The outbreaks in India are the worst seen in the country for 25 years, reports the New York Times.)

The FAO has estimated that the current outbreak is the largest seen in 70 years in Kenya and in 25 years in Somalia and Ethiopia. Pakistan officials say the outbreak there is the worst in 30 years.

Yearly records on the number and size locust outbreaks are not readily available for countries in East Africa, Keith Cressman, a senior locust forecasting officer at the FAO, tells Carbon Brief. This makes it difficult to establish if the current outbreak in East Africa is truly unprecedented.

The FAO does keep data on the number of countries reporting locust swarms each year globally. This is shown on the chart below, which includes the number of countries reporting locust plague “onset” (black), “peak” (purple) and “decline” (pink) from 1900-2019.

Countries report a plague “onset” when locusts are beginning to breed, plague “peak” when they have experienced heavy and widespread outbreaks for more than a year and plague decline when numbers begin to fall again, according to the FAO.

(The chart also shows the number of countries each year reporting locusts at low densities in yellow.)

This chart shows that 2019 was not a record-breaking year for locust outbreaks worldwide – and that the world saw much larger plagues in the 20th century than in recent times.

However, it is worth noting that the factors driving locust swarms are largely regional – and global trends in locust outbreaks would, therefore, not be expected, scientists tell Carbon Brief. (The factors driving locust swarms are discussed in more detail below.)

§ How is the current locust outbreak linked to climate conditions?

Some media reports have pointed to a link between the current outbreak and unusual storm and rainfall activity around East Africa.

To understand how the cyclones and the outbreak are linked, there is a need to first understand desert-locust biology, explains Dr Philipp Lehmann, a researcher of insects and the environment from Stockholm University.

Desert locusts are “biphasic” animals, meaning they can take on two entirely different forms, says Lehmann. In their “solitary” form, they are drab brown in colour and relatively harmless to crops. But, under certain conditions, the insects can switch into a “gregarious form” – turning electric yellow and displaying swarming behaviour.

The Arabian Peninsula – the land mass between East Africa and Asia comprising Saudi Arabia, Yemen, Oman, Kuwait, Qatar, Bahrain and the United Arab Emirates – was struck by several unusually severe cyclones between 2018 and 2019.

When the first storm – Cyclone Mekunu – hit the Arab peninsula in May 2018, it filled a vast desert near Yemen and Oman, known as the Empty Quarter, with freshwater lakes.

The moisture caused lush vegetation to grow in the usually barren environment, attracting desert locusts hunting for food into the area, Lehmann tells Carbon Brief. “The first cyclone lead to this emergence of optimal breeding grounds for the locusts.”

Desert locusts only switch to a gregarious form when they reach high enough numbers in a certain area, explains Dr Cyril Piou, a locusts researcher at France’s Agricultural Research for Development Centre (CIRAD). He tells Carbon Brief:

“Once they reach a certain density, they start to touch each other a lot and this triggers them to change their behaviour.”

By the time that the second storm – Cyclone Luban – arrived in the same region in October, the locusts had just hit a critical point where they had started to multiply rapidly, says Lehmann:

“It was actually the second cyclone that was the big problem because the locusts were at a point where they had the ability optimise their reproductive capacity to produce a new front of migrating locusts.”

Spurred on by the ample food supply provided by the cyclones’ rains, the locusts multiplied rapidly – increasing their numbers 8,000-fold in just a few months.

The unusually rainy period was then followed by a particularly mild winter, which allowed the locusts to survive in large numbers, says Lehmann.

Then, in the summer of 2019, the insects began to migrate from the Arab peninsula into the horn of Africa.

As the insects moved through East Africa, the region was hit by unusually wet conditions and more cyclones – allowing the swarms to grow even larger, says Lehmann.

Across the Horn of Africa, rainfall between October and mid-November was 300% above average. In Kenya, rainfall was up to 400% higher than average. Overall, the Horn of Africa was hit by eight cyclones in 2019, the largest number in any year since 1976.

As well as providing the conditions needed for vegetation growth, cyclones could also worsen outbreaks by providing winds for the locusts to hitch a ride on, says Lehmann:

“Winds can definitely push insects in great numbers over great distances – and at quite a small energetic cost, meaning that they have more energy available for reproduction.”

The unusual wet weather in East Africa is linked to a wider climate system known as the Indian Ocean Dipole (IOD). The dipole affects weather on both sides of the ocean, from East Africa and the Arab Peninsula to Indonesia, Papua New Guinea and Australia. (The IOD also sometimes referred to as the “Indian Niño” because of its similarities to El Niño in the Pacific.)

The dipole, which was formally identified in 1999, has three phases – positive, negative and neutral. Events typically develop in the northern hemisphere summer, peak in the autumn and then decline rapidly in winter. However, events can also stay put for extended periods. 

In a neutral dipole phase, waters around Indonesia, Papua New Guinea and Australia are warm, which causes air to rise up and fall as rain. Winds blow in a westerly direction. In this phase, the dipole has very little effect on temperatures in countries surrounding the Indian Ocean. The diagram below illustrates what occurs in a neutral phase.

During a positive phase, however, this pattern reverses. Westerly winds weaken and, sometimes, easterly winds form – dragging warm water towards the Arab Peninsula and the Horn of Africa.

This, in turn, plays a role in driving cyclones and heavy rainfall in the region. Cyclone frequency increases during a positive dipole phase because the additional warmth and  moisture brought by the climate system acts as fuel for budding storms. The diagram below illustrates the impacts of a positive IOD phase.

The IOD was in positive phase in the June to December period of both 2018 and 2019. In 2019, the dipole reached its most extreme positive level in 40 years.

As well as driving rains in East Africa, the strongly positive dipole also played a role in driving Australia’s unprecedented 2019-2020 bushfires. This is because rains shifted towards East Africa, leaving Australia with drought-like conditions.

(A recent analysis by the World Weather Attribution initiative found the conditions for fire in Australia were made “much more severe” by the positive dipole phase.)

§ Is climate change affecting the Indian Ocean Dipole?

Some people have suggested that the extreme wet weather seen during the 2018 and 2019 dipoles could be linked to climate change.

António Guterres, the UN’s secretary general, recently said in a statement:

“There is a link between climate change and the unprecedented locust crisis plaguing Ethiopia and East Africa. Warmer seas mean more cyclones generating the perfect breeding ground for locusts. This is getting worse by the day.”

The extreme positive dipole in 2019 certainly does fit into a picture of long-term change, says Dr Wenju Cai, director of the Centre for Southern Hemisphere Oceans Research at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia. He tells Carbon Brief:

“It’s highly likely that the event that we’ve just seen has a very strong warming component in it – but attributing individual events to long-term climate change is not a simple issue.”

To understand how the dipole is changing, scientists study something known as the “dipole mode index” – which is the difference between average sea-surface temperatures in the western Indian Ocean and the eastern Indian Ocean, explains Cai.

During a positive phase, temperatures in the western Indian Ocean are higher than in the eastern Indian Ocean – giving an overall positive index. The chart below shows how the monthly dipole mode index changed from January 1979 to December 2019.

The chart shows how the mode index reached its highest recorded level in the northern hemisphere winter of 2019, the time when East Africa was hit by heavy rains and unusual cyclone activity.

The dipole was also positive in 2018, when the Arab peninsula was struck by several cyclones – though this dipole was less extreme, Cai says.

Looking back over longer timescales, it is clear that the dipole mode index has been “rising up steadily”, he adds:

“There is always a role for natural variability, but we think this is due, in a large part, to climate change because this persistent trend of 60-70 years is not fully consistent with, for example, decadal variability, which would have gone up and down over a 70-year period.”

A 2009 study led by Cai found that positive phases of the dipole have increased in frequency since the 20th century. In the early 20th century, a positive dipole phase occurred roughly four times in a 30-year period. However, in the 30-year period spanning 1989-2009, there were 10 positive dipoles.

A study published this week in Nature reinforced the finding that positive dipole events are becoming more frequent. It looked at ancient coral records stretching back to 1240 and found that positive dipole events that were once rare in the ancient world and now commonplace.

Climate change could be skewing the dipole in favour of positive phases because the western Indian Ocean – near Africa – is warming at a faster rate than the eastern Indian Ocean, Cai explains:

“With the western Indian Ocean warming faster, it’s easier for the temperature difference between the west and the east to become very large. And, so, it’s much easier to have the rising air that brings rain happening in the western part of the Indian Ocean.”

Further analysis by Cai and colleagues found that uncontrolled future climate change could cause positive dipole events to increase by a factor of three by 2099, when compared to the period 1900-1999.

Even if global temperature rise is limited to 1.5C – the aspirational target set by the Paris Agreement – the number of positive dipole events could double, when compared to the pre-industrial era, according to another study.

Such increases are likely to bring about more frequent and intense cyclones, says Dr Agus Sontoso, a senior research associate at the Climate Change Research Centre at the University of New South Wales in Sydney. He tells Carbon Brief:

“The Indian Ocean itself is warming everywhere. So it’s reasonable to think that under climate change, we expect to see more frequent, if not stronger, cyclones in the region.”

§ What other factors are playing a role in the current locust outbreak?

Though the rainy weather played a role in driving the current locust outbreak, there are several factors that could have contributed to the insects’ spread, scientists tell Carbon Brief.

One major factor is that countries did not act quickly enough to prevent the locusts from spreading in 2018, says Piou:

“This specific crisis is also due to the fact there was no preventative management in many of the places where the rain fell. Saying it’s directly linked to climate change is too simple – it’s forgetting that we should have been preventing the outbreak in the first place.”

According to the FAO, the best way to stop locusts from spreading is to spray small concentrated areas with pesticides. This is most effective early on in an outbreak, when the insects are still multiplying, says Piou.

The responsibility of controlling locust numbers falls to national governments – with international organisations stepping in during outbreak and crisis situations.

The insects first started to multiply in the south of the Arab peninsula, a region experiencing political instability and conflict. This may have thwarted early detection and prevention methods, says Lehmann:

“There’s actually some studies on this – though not in an African context. I’ve worked a lot with the Colorado potato beetle, which arrived in Europe during the first world war. Before the war, it was contained by early prevention efforts. During the first world war, people had other things on their mind – the potato beetle arrived, established itself and started spreading. Then, during the interwar period of the 20s and 30s, the spread was quite contained. But during the second world war, there was the biggest spread that we’ve ever seen.”

It is worth noting that other parts of Africa, including Niger and Mauritania, periodically experience heavy rains, but do not see outbreaks on the same scale, says Piou:

“The prevention system is working in some other parts of the world. For example, in western Africa, they experience heavy rains that arrive and create some good conditions for the desert locust to breed and multiply – but they don’t see such large outbreaks.”

Another possible driver influencing the current outbreak could be an increase in irrigated agriculture in parts of East Africa, says Lehmann:

“People live year-round in these semi-arid areas and you need irrigation in order to maintain crops. This also creates stopping points for a swarm.”

Overall, it is likely that several factors worked in tandem to make the current outbreak so extreme, says Lehmann:

“I think there can be several reasons for why an outbreak starts and, in many ways, it’s like the stars have to align – and, when they do, something very bad happens.”

§ Will locust outbreaks become more common in East Africa in the future?

Though studies suggest that the wet conditions seen in Africa during the winter 2019 are likely to become more frequent with climate change, it is less clear if there will be a corresponding increase in locust outbreaks, scientists tell Carbon Brief.

This is because many factors influence the chance of an outbreak, including human factors such as taking early preventative measures, says Piou:

“If there are more cyclones in the Arab peninsula, then eventually, yes, there could be more outbreaks. But still, in any case, the main message should be to be prepared for that with a preventative system.”

It is likely that locust outbreaks will “stay bad” in East Africa, says Lehmann:

“I think they will stay bad – as they are. I base this simply on the fact that locusts are cyclic and there have been outbreaks for thousands of years. If they return, I think there can be several reasons for why an outbreak starts.”

The “jury is still out” on how future temperature rise in the East African region could affect outbreaks, he adds.

(According to Carbon Brief analysis, temperatures around the horn of Africa could increase 1.6-4.6C by 2100, depending on the rate of future greenhouse gas emissions.)

Research suggests that desert locusts do prefer hotter temperatures, says Piou:

“The perfect temperature for a desert locust to multiply is around 35C during the day. And even below this temperature, they display warming up behaviours to reach a body temperature of 40C, which is the optimal temperature for their metabolism.”

However, despite hotter temperatures giving the insects a physiological advantage, it is not yet clear if this would influence their swarming behaviour, says Lehmann.

Overall, climate change is expected to make weather conditions in East Africa less predictable – which is likely to benefit insect pests, he adds:

“I don’t know if it’s the locusts, per se, or some other pests that will respond – but insect pests generally thrive in unpredictable environments.”