Climate science at a glance

• Extreme and unusual weather - including heavy rains and eight tropical cyclones in 2019 - created the wet conditions that fueled the locust outbreak.
• Climate change may be linked to the unusual circulation patterns in the Indian Ocean behind this extreme weather.
• As greenhouse gases continue to heat the ocean and the atmosphere, extreme events caused by the IOD are predicted to become increasingly common.

Climate signals breakdown

Climate signals #1 and #2: Large scale circulation change and sea surface temperature increase

The Horn of Africa was hit by eight cyclones in 2019, the largest number in any year since 1976. In Mozambique, two cyclones hit within 6 weeks of each other (Cyclones Idai and Kenneth), affecting about 1.85 million people. The unusually active cyclone season in East Africa in 2019 is linked to an ocean circulation pattern known as the Indian Ocean Dipole (IOD).

What is the Indian Ocean Dipole?

The ocean circulation pattern known as the Indian Ocean Dipole (IOD) measures the difference in water temperature between opposite sides of the Indian Ocean. It is a primary driver of climate conditions stretching from Africa to Australia.

Tropical cyclones are fueled by available heat, and during the positive phase of the IOD, sea surface temperatures in the tropical western Indian Ocean are above average. In addition, climate change is contributing to sea surface temperature increases in the tropical regions where hurricanes form.

The West Indian Ocean, including the Arabian Sea, was warmer than usual during the last two seasons. This is largely due to a phenomenon called Indian Ocean Dipole, and also due to the rising ocean temperatures associated with global warming.

Roxy Koll Mathew, Indian Institute of Tropical Meteorology in Pune

The positive phases of the IOD are becoming more common, and scientists believe climate change is responsible. Studies have found that strongly positive phases of the IOD have happened more often in recent decades, and that climate change is behind the increase.

Unusually positive IOD events could happen nearly three times more often this century if emissions continue to rise, according to a 2014 study. A separate study also found they would be twice as likely to happen even with only 1.5°C of warming - which is only a little more than has already been seen.

Increased extremely positive IOD years would likely bring flooding and cyclones like those seen in 2019 to already vulnerable and food insecure regions. Wet conditions could also lead to worse locust outbreaks - in a worst-case scenario they could damage the livelihoods of one tenth of the world's population, according to the FAO.

As Desert Locust are fully integrated with nature, weather and environmental conditions have dramatic impacts on locust numbers and migration. Historically, heavy rains associated with cyclones that form in the Indian Ocean and make landfall in the Arabian Peninsula and the Horn of Africa have led to Desert Locust plagues.

In the past few years, there has been a significant increase in the frequency of such cyclones at the beginning and end of the summer period. For example, there were 8 cyclones in 2019 when in most years there are only one or two. Three cyclones in 2018 and two in 2019 have contributed to the current Desert Locust upsurge in the Horn of Africa where large and numerous swarms are present in Ethiopia, Somalia and Kenya.

Keith Cressman, Senior Locust Forecasting Officer, FAO

Observations consistent with climate signal #1 and #2

• The positive phase of the IOD in 2019 was the strongest for six decades. These conditions led to severe rainfall and flooding in East Africa, as well as contributing to the unusually dry conditions in Australia that drove the current bushfires.
• There were eight tropical cyclones during the 2019 North Indian Ocean cyclone season, compared to most years that usually have one or two.

Climate signal #3: Extreme precipitation increase

Independent of changes to large scale circulation patterns, climate change increases extreme rainfall. This is because warmer temperatures increase evaporation of moisture into the air, and warmer air contains more water vapor. Warmer, water-laden air can then dump more precipitation during storms, including tropical cyclones, increasing flood risk.

Locusts thrive in wet conditions, and outbreaks often follow extreme rain events. Heavy rain leads to growth of vegetation in arid areas, providing locusts with the conditions needed to develop and reproduce, according to the World Meteorological Organization (WMO).

Observations consistent with climate signal #3

• The October to mid-November rainfall was up to 300 percent above average; in Kenya, the rainfall was up to 400 percent above average.