Monday, Mar 25, 2019

Sahara growing by more than 10% over the last century

Sahara growing by more than 10% over the last century
29 Mar

Global warming is causing the Sahara desert to grow, new research suggests.

Scientists have found that the world's largest desert has expanded by more than ten per cent over the last 100 years.

The study suggests the rest of the world's deserts could be expanding too as widespread climate change continues to heat up the planet.

'Our results are specific to the Sahara, but they likely have implications for the world's other deserts,' said Sumant Nigam, a professor of atmospheric and oceanic science at the University of Maryland.

The researchers analysed seasonal rainfall data throughout Africa from 1920 to 2013.

The team found that areas around the Sahara that were once non-desert regions could now be classified as deserts.

Deserts are defined as places with a very low average rainfall, usually four inches (100 mm) or less, and many areas around the Sahara now fall below this threshold.

During the summer months, the expansion of the Sahara was most noticeable, researchers found.

In these months, it resulted in a nearly 16 per cent increase in the desert's average area over the 93-year span covered by the study.

The senior author of this study, Professor Nigam added: 'Deserts generally form in the subtropics because of the Hadley circulation, through which air rises at the equator and descends in the subtropics.

'Climate change is likely to widen the Hadley circulation, causing northward advance of the subtropical deserts.

'The southward creep of the Sahara however suggests that additional mechanisms are at work as well, including climate cycles such as the AMO.'

Researchers claim that the reason for the expansion of the Sahara is a combination of human-influenced factors and natural change.

The Atlantic Multidecadal Oscillation (AMO), which alters climate patterns in the region, works on a 50- to 70-year cycle.

The Pacific Decadal Oscillation (PDO), marked by temperature fluctuations in the northern Pacific Ocean on a scale of 40 to 60 years, also plays a role in changing temperatures.

Although the pattern varied seasonally, the expansion was steady and undeniable, researchers found.

The southern edge of the Sahara borders the Sahel region, the semi-arid transition zone that lies south of the Sahara.

Researchers found the Sahara expands as the Sahel retreats, disrupting the region's fragile grassland ecosystems and human societies.

Lake Chad, which sits in the centre of this conflicted transition zone, is used to judge the changing conditions in the Sahel.

'The Chad Basin falls in the region where the Sahara has crept southward. And the lake is drying out,' Professor Nigam explained.

'It's a very visible footprint of reduced rainfall not just locally, but across the whole region. It's an integrator of declining water arrivals in the expansive Chad Basin.'

Warm phases of the AMO are linked to increased rainfall in the Sahel, while the opposite is true for the cold phase.

Researchers used statistical methods to remove the effects of the AMO and PDO in order to look at direct human influences.

These natural climate cycles accounted for about two-thirds of the total observed expansion of the Sahara. The remaining one-third can be attributed to climate change.

'Many previous studies have documented trends in rainfall in the Sahara and Sahel', said Natalie Thomas, a graduate student in atmospheric and oceanic science at the University of Maryland and lead author of the research paper.

'But our paper is unique, in that we use these trends to infer changes in the desert expanse on the century timescale,' she said.

This study shows that the changing weather is also reducing the amount of fertile land where crops can be grown.

This could have devastating impacts on both local and global communities.

'The trends in Africa of hot summers getting hotter and rainy seasons drying out are linked with factors that include increasing greenhouse gases and aerosols in the atmosphere,' said Ming Cai, a program director in the National Science Foundation's Division of Atmospheric and Geospace Sciences, which funded the research.

'These trends also have a devastating effect on the lives of African people, who depend on agriculture-based economies.'

The study was published online March 29, 2018, in the Journal of Climate

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