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Global Dimming - Worse than Global Warming?

Imagine waking up to a world, where the climate is reversed, the global north becomes warmer and the global south cooler. It may seem like a wildly disturbing possibility, wherein decreasing levels of air pollution, thanks to effective climate policies, would have resulted in a paradoxical, rapid increase in global warming. So, is there really no solution to the climate crisis? Is that plausible? Well, the only way to find a way out of this loop is to understand the phenomena of Global Dimming; as it sits brushed away from the mainstream, a distant cousin of global warming that no one dares to talk about.

Planet Earth has a sensitive climate; even a small change in the atmosphere can trigger dangers for humans: some impacts have already manifested - cyclones, volcanic eruptions, floods, and earthquakes. This uncertainty shouldn’t be a cause of fear rather the beginning of an acceptance towards a world where the more we try to stake claim, the more we have to lose, both in resources and lives. The ongoing global pandemic is the latest result of this uncertain future that has been passed on to us.

Tracing the Process

Under normal conditions, the Sun, our only source of heat, emits solar radiation. Approximately 26% of the incoming solar radiation, or insolation, is reflected back into space by the Earth’s atmosphere and clouds. Only 19% is absorbed by the atmosphere. Insolation is of shorter wavelengths compared to the outgoing solar radiations. This process of give and take forms a balance called the ‘heat budget’ without which extreme temperatures would cause excessive cooling and warming on the planet.

Aerosols like fog, mist, dust particles, volcanic ash and deodorant sprays are significant contributors to Air Pollution. When they enter the atmosphere they reflect back the incoming solar radiation into space, just like clouds do, acting as a reflective piece of mirror. This destabilizes the heat budget, resulting in cooling, which results in ‘global dimming’.

Global dimming is caused by a drop in the amount of solar radiation reaching the surface of our planet, which as a result blocks sunlight, and cools the atmosphere. This phenomenon was first observed in the 1950s when Gerald Stanhill, an English scientist working in Israel, studied the decline in a few scientific papers, and coined the term. Even then, the scientific community at large failed to take notice of the issue. Measurements from the 1960s to the early 1990s backed up by a range of data and independent studies proved the presence of the phenomenon. However, some say (Bressan,2015) that the issue goes way back to 1783, when a volcanic eruption in Iceland made scientists open their eyes to the possibility of smoke and ash blocking sunlight leading to cold weather in Europe. The connection was made clearer in the 20th century; surprisingly this change in Iceland even led to the most catastrophic famine in the nation’s history.

Historical Trajectories

During World War – II the new field of Aerosol Science was developed to understand the spread of aerosol particles from wartime explosions and its impact on climate. Furthermore, some scientists warned that a nuclear war could lead to a nuclear winter. It has also been linked that the extinction of dinosaurs was due to a nuclear winter induced by an asteroid hitting Earth, which led to Global Dimming; that cooled the atmosphere and led to their demise. The clearest evidence came when Mount Pinatubo in the Philippines erupted out of the blue, releasing 20 million tons of Sulfur Dioxide, and creating a haze of Sulfate Aerosols. This event was investigated by NASA scientists, who deduced that a half degree drop in global temperatures especially in the northern latitudes was to be expected in the coming decade as a result of this eruption. Interestingly, that is what happened, convincing the scientific community by the mid-1990s that global dimming must not be ignored anymore.

All this uproar for a phenomenon that has been observed in almost all parts of the world today was just limited to the Global North back then, maybe due to the rapid advancements in industrialization and urbanization being witnessed there. It was not until the 1960s, that a cumulative mixture of industrial pollution, haze, smog and shifting cultivation made the Global South bear witness to it as well.

Closer home, an Indian scientist, Dr Veerabhadran Ramanathan has documented how pollution was severely dimming areas of the Indian Ocean. In his famous study called Project INDOEX, he found that a huge brown ash cloud generated from pollution streams traveling airborne over Indian Ocean from India to the Maldives was a cause of concern. They were creating a nearly 3 km thick layer of toxic aerosols which was blocking sunlight reaching the ocean. With mounting evidence, NASA launched satellite Aqua in 2002 which gathered data confirming aerosol pollution was cooling the climate by more than a degree Celsius. This showed that the warming of the planet due to global warming, was consistently being offset by the cooling effect from global dimming.

It has also been theorized since 2002 that climate change may increase volcanic activity. Globally, on average, there are around 60 to 80 volcanic eruptions annually. According to a recent study that analyzed volcanic fallout records in six Arctic ice cores, one of the largest volcanic eruptions of the past 2,500 years occurred in early 43 B.C.E. and it was among the coldest years of the recent millennia in the Northern Hemisphere at the start of one of the coldest decades (Connolly, 2020) .

The same study suggested that this high-latitude volcanic eruption led to pronounced changes in hydro-climate, including colder seasonal temperatures in specific Mediterranean regions during the two-year period following it, thus the link between volcanic eruptions and cooling on account of global dimming may have a longer history than anticipated. While we cannot rule out the possibility that natural variations in the Earth’s climate may have contributed to this, the effects are so closely related to air pollution that human activity seems to be largely responsible. Thus, pollution from millions of vehicles and airplane contrails not only causes global warming, but also global dimming. Hence it becomes imperative that emphasis be laid not just on curbing the intensity of polluted air but also taking note of the composition of polluting particles in it.

A series of Consequences

Global Dimming has three-fold economic, environmental and social implications. It can affect the solar energy industry; lack of sunlight leads to delay in charging of solar power dependent units. The environmental costs in terms of diminishing agricultural productivity and acid rain are going to cause damage in certain areas causing social distress, unrest and diseases that are respiratory in nature, much of which seems to be already unfolding. It can also lead to changes in the rainfall pattern; causing water in the Northern Hemisphere to become colder, which means slow evaporation and fewer water droplets. This in turn reduces the amount of rain worldwide, which is a cause of concern for the Asian Monsoon that is responsible for 50 percent of the world’s annual rainfall.

It had already been established that the drought and famine of Sahel that killed thousands of innocent people in sub-Saharan Africa during the 1970s was largely due to global dimming (Shah, 2005). A 2007 NASA-sponsored satellite-based study sheds light on the puzzling observations that the amount of sunlight reaching Earth’s surface which had been steadily declining in recent decades, began to reverse around 1990. This switch from a “global dimming” trend to a “global brightening” trend happened just as global aerosol levels started to decline in the atmosphere, due to strict policy measures taken in the Global North. From here on the problem hasn’t gone away, it largely shifted location to Global South.

A micro look at India

A specific look at India suggests a grim scenario with a 6.3% decline per decade for the national capital, New Delhi. Activities like biomass burning are causing dimming. The spread of urban areas and industrial activity in all metros and in fast-developing cities such as Nagpur and Pune have contributed as well. Delhi is influenced by heavy injections of dust loads blown over from deserts in the western parts of the country, particularly during pre-monsoon season. Delhi and parts of North India also get dust particles transported from the Sahara.

The way ahead

There might still be some time before we say a final goodbye to sunshine altogether. Sometime in the future we may see stricter air pollution regulations in the Global South. With The Pandemic we can already observe the short term positive impact it can have on our environment. With lesser traffic and industrial activity decreasing emissions. On the other hand, decrease in aerosol will allow more sunlight, more heat in the environment, this will surely accelerate climate change and is a cause of concern.

In the long run, a turn to global brightening can be expected but this should be accompanied with strict reduction in greenhouse gas emissions, if we want to tackle both problems at once. It is a welcome sign that the developing countries in the world are changing their energy mixes to renewables rapidly and realizing that pollution is killing their people and slowing their economies. Global dimming will decrease over the coming decades. There will still be airplane contrails, forest fires, volcanoes and the like to deal with, but reduction in fossil fuel consumption should balance the system. If even this threat doesn’t make human beings stop exploitation of nature, then perhaps we surely are going to wake up to a world, where not only the climate is reversed, but also our fate. Humanity may be looking at its last chance to redeem itself from all that is yet to come. Sometimes fear can be good; it calls for action and mobilization to solve the problems plaguing this planet even if it is a Global Pandemic, or perhaps another extinction. Who knows?


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A Night Owl original, this piece has been authored by Yastika Sharma.


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