Release causes delayed rainfall

Clouds cling to the hillside behind a tea plantation near Kerala, India. Monsoon rainfall affects agricultural activity in the region in particular, but new studies show that the start of the monsoon season is now delayed by more than four days due to past emissions. Credits: rajib |

The Earth shows many signs of human influence, from pre-industrial warming above temperatures to rising oceans. In addition to that list, the human impact on the timing of the Earth’s water cycle is now revealed by a new study led by researchers at the Pacific Northwest National Laboratory of the US Department of Energy.

A study published in the journal this week Natural climate changePeel off the layer of climatological noise to reveal a clear signal: 1979-2019, Greenhouse gas The reduction in human-generated aerosols has caused a delay of approximately four days in seasonal rainfall in tropical lands and the Sahel. Delays can mean delayed crop production, worsening heat waves, and worsening wildfires, among other consequences.

Global warming “What we’ve seen so far is already confidently attributed to human activity,” said atmospheric scientist Ruby Leon, who co-authored the study. This study does show that the subsequent onset of monsoon rainfall has already emerged, coupled with future warming predicted by climate models. “

More moisture

Ironically, rain delays are caused by an increasingly moist atmosphere. As greenhouse gases warm the surface of the earth, more water vapor is released into the atmosphere. This added moisture increases the amount of energy needed to heat the atmosphere as spring transitions to summer and can change the timing of the rainy season.

“The more water vapor in the atmosphere, the more sea-like it becomes,” said Fengfei Song, a geoscientist and lead author. “And we know that the ocean takes longer to warm up than the atmosphere. Moisture means that the atmosphere takes longer to absorb energy and produce rainfall. I will. “

Like particulate matter produced by the burning of fossil fuels, human-produced aerosols tend to reflect sunlight. They cool the atmosphere and reduce the warming tendency caused by greenhouse gases.

However, as aerosol concentrations continue to decline from efforts to improve air quality, some of their cooling effects are lost, and studies have shown that both warming and delayed monsoon rainfall have escalated in recent decades. I will.

If aerosol concentrations continue to decline and greenhouse gas concentrations continue to rise, delays will be extended in the future, Leung said. By the end of the century, the authors of the study predict that the rainy season could be delayed by more than five days in the northern tropical lands and more than eight days in the Sahel.

“In monsoon regions such as India, where there is an agricultural economy, delayed start of summer rainfall can ruin crop production and cause large numbers of crops unless farmers recognize and adapt to long-term changes in the face of high-level changes. It can endanger people’s lives. The fluctuating monsoon start date. “

Release causes delayed rainfall

Water vapor is an important element of the Earth’s water cycle. Steam rises from the ocean, among other sources, and air currents move its moisture throughout the Earth’s atmosphere. However, adding too much water to the atmosphere can change the timing of the rainy season. Credits: Quangpraha |

What is the water cycle?

Most of the earth’s water is in the ocean. Sunlight warms the sea, and some of its water rises from the surface as steam.It water vaporWhen encountering rising air, along with water generated from plants and water evaporated from soil, condensation can form clouds and rainfall.

Updrafts are strongest in the tropics, where solar radiation is strongest. If you look at our planet from space, you can see the products of these powerful, moist air currents. The Earth’s characteristic rain belt is active in storms and showers and surrounds the center of the planet.

As the seasons change and the sun moves between hemispheres, the rainband moves. When the band reaches land, it marks the beginning of the monsoon season and provides ample water for both the rainforest and the large numbers of people living in and near it.

In a world warmed by human activity, a moist atmosphere stores more energy, delaying the movement of rainbands and the onset of monsoon rainfall.

Signal in noise

Researchers first predicted rain delays through climate modeling about 10 years ago. However, it turned out to be difficult to point out the delay in the observation record. It’s not just about measuring global precipitation. According to Leung, there are two difficulties.

First, unraveling the anthropogenic effects of the daily or yearly changes in the Earth’s climate is a daunting task. Consider trying to pinpoint the characteristic audio wave of a single voice in a recording filled with background chattering.

“Temperature and precipitation are not the same every year,” Leung said. “There is a great deal of variability.”

The second challenge is Data collection.. When historical records of world temperatures are long-standing, data-rich and directly measured, world precipitation records are relatively short and plagued by uncertainty.

Satellites indirectly measure global precipitation by detecting the energy reflected by clouds and raindrops. This introduces some uncertainty. The spread of satellites began in the late 1970s and set a record for only a few decades.

As global precipitation data became more and more available and climate models became more robust, delay signals buried in the noise of climate change became apparent. Using eight observational datasets combined with 243 simulations performed through multiple models, the authors of the study were able to finally show that rainfall was actually delayed in the spring of the Northern Hemisphere.

Studies show that many of the seasonal changes are the result of post-WWII economic development, with increased greenhouse gas emissions and subsequently reduced aerosol emissions since the 1980s. did. But human activity was not the only factor behind the delay. Decades of sea surface temperature fluctuations may also contribute to seasonal changes.

Researchers Reconstruct Precipitation Seasonality in Central China Over the Last Century

For more information:
Fengfei Song et al, The emergence of seasonal delays in tropical rainfall from 1979 to 2019, Natural climate change (2021). DOI: 10.1038 / s41558-021-01066-x

Quote: Emissions cause delays in rainfall (25 June 2021) 25 June 2021

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