Earth’s tropical rain belt produces humid and precipitous ecosystems around the world. Image courtesy of Pixabay.
By Helen Gloege ’23
Staff Writer
Climate change will cause an uneven shift in the Earth’s tropical rain belt, an area that covers almost two-thirds of the world, stated a recent study published in scientific journal Nature Climate Change.
The tropical rain belt, also known as the Intertropical Convergence Zone, contains equatorial areas considered the warmest in the world. The belt is a meeting point for trade winds from the planet’s Northern and Southern hemispheres, which bring in humidity and precipitation. The rain belt oscillates annually from the Northern Hemisphere to the Southern Hemisphere tropics in movement with the sun.
The study entitled “Zonally contrasting shifts of the tropical rain belt in response to climate change” used projections from 27 state-of-the-art climate models. The study shows climate change will cause the position of the rain belt to move in opposite directions in two longitudinal sectors. Based on the model, the shift will be complete between 2075 and 2100. The weakening of the Gulf Stream current and deep-water formation in the North Atlantic will likely cause a southward shift in the tropical rain belt in the Western Hemisphere over the Pacific and Atlantic oceans. In the Eastern Hemisphere over Africa and the Indian Ocean, the rain belt will shift north.
The shift in the belt will have cascading effects on food production and water availability around the world. The shift in the Eastern Hemisphere will likely result in drought stress over Madagascar and intensified flooding over southern India.
The country of Madagascar is already seeing the impacts of a continuous drought. In December 2020, the country only had one day of rain. Madagascar has the tenth highest rate of stunting, as almost half of children under 5 years old suffer from chronic malnutrition. Asia is likely to experience the effects of the moving rain belt faster than the rest of the world because of compounding glacier melting in the Himalayas and loss of snow cover in northern areas brought on by climate change. The tropical rain belt is what is known to drive Indian monsoons and the southern peninsula in India is likely to witness impacts in the form of frequent severe flooding. Parts of southern India are already witnessing extreme flooding and numerous episodes of extreme rainfall events. In the Western Hemisphere, there will be increased drought stress in Central Asia.
Additional changes are already occurring with the rain belt shift. The tropics are found to be increasing in size by at least 30 miles per decade. The boundary of the tropics is marked by the Tropic of Cancer to the north and the Tropic of Capricorn to the south. The lines are at about 23 degrees north and south, determined by where the sun lies directly overhead on the solstices. A little over a decade ago, scientists noticed the dry belt appeared to be enlarging. The dry edges of the tropics are expanding as subtropics push north and south. This resulted in drier weather in places like the Mediterranean and the development of a smaller equatorial region with heavy rains that are contracting known as the “tropic squeeze.” The boundary between decreased precipitation and increased precipitation has been pushed farther north. This causes countries as far north as Germany and Britain to become drier while already dry Mediterranean countries are seeing the changes. It is likely that the changes in the sea surface temperature are causing half of the shift.
Similar changes are occurring in the Sahara Desert which already covers 3.6 million square miles, an area almost as large as the U.S. The desert’s borders are defined by rainfall. It is estimated that the desert has grown 10 percent bigger since 1920. Across most of the Sahara, the change is only tens of miles. In other countries, the change is more pronounced and dramatic. Libya went from lacking desert in 1920 to being mostly desert in 2013 with the line advancing 500 miles. Lake Chad on the southern edge of the Sahara shrank from 9,600 square miles in the 1970s to less than 770 square miles in the 1990s in part due to reduced rainfall in the region. About two-thirds of the change might be due to natural climate cycles from the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation that help determine rainfall. Yet, the remaining one-third is likely due to climate change, as the northern edges of the desert appear to be moving because of the poleward movement of the tropics.