Reducing The Prevalence Of Short-Lived Pollutants Would Be A Massive Win For The Planet.
Carbon dioxide is the belle of the climate ball. And rightly so; it’s Earth’s most important greenhouse gas and the main cause of the climate crisis. Humanity emits tens of billions of CO2 every year and has cumulatively emitted nearly two trillion tons of carbon dioxide since 1750. Even though carbon dioxide still only occupies about 0.4% of the atmosphere, the atmospheric concentration of carbon dioxide has risen by about 50% thanks to human activity.
Carbon dioxide might be the most famous compound in the climate crisis, but if we want to quickly reduce the greenhouse effect that principally warms our planet, there are other manmade agents of destruction that we cannot forget.
Carbon dioxide is a long-lived climate pollutant (LLCP). These pollutants usually remain in the atmosphere for 100 years or longer. Given their longer lifespan, they have more time to warm our planet. Carbon dioxide drives just over half of today’s global warming, but that leaves a lot unaccounted for.
Much of the remainder comes from short-lived climate pollutants (SLCPs), which usually leave the atmosphere within 20 years. These pollutants don’t last as long, but they make a statement while they’re around. As society’s addiction to fossil fuels collides with the planet’s natural processes and its ability to correct humanity’s negligence, SLCPs cannot be forgotten.
There are four main SLCPs to know: black carbon, methane, tropospheric ozone, and hydrofluorocarbons (HFCs). Methane is mainly emitted from fossil fuel production, agriculture, and waste decay in landfills. Hydrofluorocarbons are emitted from some manmade activities that involve heat transfer, including refrigeration and air conditioning. Black carbon is emitted from a combination of everyday household activities and industrial activities such as biomass for cookstoves, diesel exhausts, and black coal. Tropospheric ozone is not emitted directly; it forms from a mix of methane, carbon monoxide, nitrogen oxide, and volatile organic compounds.
Methane and HFCs tend to disperse more freely in the atmosphere (like carbon dioxide), meaning their impacts are more global. Black carbon and tropospheric ozone tend to be more atmospherically concentrated, so their impacts are more local and regional.
What these SLCPs share is a major mean streak. They’re very potent in the atmosphere and awful for human health. For instance, over a 20-year time horizon, methane is 86 times as potent as carbon dioxide as a greenhouse gas, while HFCs are hundreds to thousands of times more potent than carbon dioxide. Collectively, SLCPs are responsible for up to 45% of current global warming. Meanwhile, they are a main contributor to the nearly 10 million annual deaths that come from air pollution.
It’s this combination of short lifespan and high impact that makes SLCP mitigation an attractive and quick climate solution that can address many overlapping problems at once while biding us more time to address some of the thornier aspects of the climate crisis.
Methane is a powerful greenhouse gas emitted by both natural and manmade sources. It remains in the atmosphere for only about 12 years on average but, as previously mentioned, warms the planet 86 times as much as carbon dioxide over a 20-year period. It’s generally considered the second-most important greenhouse gas after carbon dioxide, and emissions are growing at an increasing rate. The Environmental Defense Fund estimates that methane accounts for at least 25% of today’s global warming.
One key variable is whether those increases in methane concentrations have been driven by nature-based sources or manmade processes. Manmade sources of methane include leakage from fossil fuel infrastructure (like pipelines), landfills, various industrial processes, and cattle production. Of these manmade emissions, roughly 35% comes from leaks of methane from fossil fuels, mainly through extraction and transport, 20% comes from waste, and over 40% comes from agriculture.
The latter source—cow burps—is particularly noteworthy. There are about one billion cows around the world, and they collectively generate about 40% of global methane emissions. But methane is also released by natural sources like Arctic permafrost, and many scientists are worried that manmade climate change will induce tipping points like permafrost melt that will amplify methane emissions from these sources in a way that can’t be as easily controlled. Global warming will also quicken the decomposition of organic material, which emits methane if done in water. If tropical wetlands become wetter and warmer, which is a distinct possibility, more methane emissions are possible. Recent research has validated these concerns, showing that biological sources of methane are a primary driver of post-2006 increases in methane emissions.
In essence, by destabilizing natural processes, we are limiting nature’s ability to clean up our mess. Plus, we’re unleashing a degree of uncertainty that challenges our ability to combat the climate crisis, no matter how much technological advancement we achieve over the coming decades. Tipping points related to methane, like permafrost melt, represent just some of the many harrowing examples of this trend.
The World Meteorological Organization (WMO) recently released a sobering study on the same day as a new U.N. report that says the world’s governments haven’t committed to cutting enough carbon emissions, putting the world on track for a 2.5°C increase in global temperatures by the end of the century. According to the WMO report, relative to preindustrial times, carbon dioxide concentrations in 2021 were 49% higher, methane levels were 162% higher, and nitrous oxide levels were 24% higher.
Just before 2021’s COP26 gathering, the United States and European Union took the lead in promoting the Global Methane Pledge, which targets a 30% reduction in atmospheric methane by 2030. It has over 100 signatories representing nearly 50% of global anthropogenic methane emissions and over two-thirds of the global GDP. But there remain many notable holdouts, including China.
The aforementioned WMO report estimates that lowering atmospheric methane levels by 30% by 2030, which is the stated goal of the Global Methane Pledge, could shave 0.2°C off the rise in temperatures that would otherwise take place. Put differently, International Energy Agency Executive Director Fatih Birol has said that cutting global methane emissions from human activities by 30% by the end of the 2020s “would have the same effect on global warming by 2050 as shifting the entire transport sector to net zero CO2 emissions.”
The U.N. estimates that nearly half of the roughly 380 million metric tons of methane released by human activities annually can be cut this decade with available and largely cost-effective methods mostly related to agricultural production and fossil fuel production. Doing so would avoid nearly 0.3°C of warming by the 2040s.
Methane frankly doesn’t get the level of worldwide scrutiny it deserves both as an agent of atmospheric chaos and as a way for us to quickly abate a lot of global warming. And similarly to decarbonization, methane reductions would bring a plethora of ancillary benefits that have nothing to do with the climate crisis.
Excessive UV radiation poses a number of threats, from skin cancer to immune system suppression to crop damage and widespread environmental harm. In 1985, scientists working in Antarctica made a startling discovery that incited global panic: the stratospheric ozone layer, which protects Earth from excessive UV radiation, was thinning rapidly. The cause of this trend was clear: chlorofluorocarbons (CFCs). These chemicals were present in everything from aerosol cans (like hairspray) to refrigerators to solvents. Left unchecked, scientists worried that CFCs would destroy the ozone layer within a few decades. Their discovery quickly galvanized international negotiations on a global treaty to ban CFCs. This led to the Montreal Protocol, which was agreed upon and opened for signature in 1987 before going into force in 1989.
The Montreal Protocol has often been hailed as the most successful international agreement in history. CFC consumption declined from over 800,000 metric tons per year in the 1980s to an estimated 156 metric tons in 2014. Experts estimate that by 2050, the ozone layer will be as strong as was in 1980.
Since the Montreal Protocol came into effect, global consumption of ozone-depleting substances has fallen mightily. But the global need for what these substances provide - namely heat transfer - hasn’t abated. As such, hydroflurocarbons (HFCs) have largely replaced CFCs. Like CFCs, HFCs are a group of industrial chemicals primarily used for cooling and refrigeration. Many HFCs are powerful greenhouse gases, and even though they only last between 15 and 29 years in the atmosphere, they pack quite a warming punch when they’re high in the skies. The most abundant HFC is 3,790 times more damaging to the climate over a 20-year period than carbon dioxide. That’s not a typo.
While HFCs don’t deplete the ozone layer (since they lack ozone-depleting chlorine), their warming potential nonetheless makes them very dangerous. Nearly 140 countries have ratified the Kigali Amendment to the Montreal Protocol, which calls for a gradual reduction in global HFC use (with more time given to poorer and warmer countries).
HFCs are a sensitive climate topic given the lack of cost-effective alternatives for developing countries that will depend on substances like HFCs both to grow their economies and to mitigate the disproportionate impacts they will suffer from climate change. But alternatives do exist, and global cooperation (and assistance from wealthier countries) could go a long way toward lowering HFC emissions and curtailing the greenhouse effect.
Black carbon is formed through the incomplete combustion of fossil fuels, biofuel, and biomass, primarily via diesel engines, cook stoves, wood burning, and forest fires. Think of the dark clouds of black soot you might see trailing a truck on a congested road. That soot has a whole lot of black carbon. It’s a component of fine particulate matter (PM2.5), the leading environmental cause of poor human health and premature deaths, and has a number of effects on the climate.
Today, black carbon tends to come from developing countries with weaker air quality regulations. Asia, Africa, and Latin America contribute a share of global black carbon emissions that is disproportionate to their share of the global population and global GDP. In contrast, black carbon emissions have been decreasing over the past few decades in many developed countries due in large part to stricter air quality regulations.
Black carbon presents a number of climate impacts, from accelerating snow and ice melt (by making those surfaces darker) to altering regional weather effects (largely by affecting cloud formation) to impairing plant health. Black carbon only resides in the atmosphere for four to 12 days on average, but it has a warming impact 460-1,500 times stronger than carbon dioxide, making it a prime candidate for global warming mitigation. Some scientists view it as the second-most important substance behind carbon dioxide in regards to global warming and perhaps the fastest way to quickly reduce warming.
Multiple simple interventions can greatly reduce global black carbon emissions. Replacing traditional cooking stoves with clean burning ones, along with eliminating kerosene lamps, would cut household energy emissions, which account for about half of black carbon emissions. Other solutions include banning open-field burning of agricultural and municipal waste and moving away from diesel vehicles.
Transportation and household energy needs account for the vast majority of black carbon emissions, so many of the same solutions that will decarbonize the global economy will also quell black carbon’s impacts. If you care about saving the polar bears, there might be no easier way to protect the ice they depend on than keeping as much black carbon out of the air as possible.
Ozone is a reactive gas that exists in two layers of the atmosphere: the troposphere (up to 15 km above the Earth’s surface) and the stratosphere (above the troposphere). In the stratosphere, ozone is critical. There, it protects life on Earth from excessive UV radiation. In the troposphere, ozone is harmful. Tropospheric ozone is a potent greenhouse gas and air pollutant that harms both human and ecosystem health.
Tropospheric ozone does not have any direct sources. Rather, it forms from the direct interaction of sunlight with atmospheric pollutants such as carbon monoxide, nitrous oxide, and various volatile organic compounds such as methane. Tropospheric ozone directly warms the atmosphere and causes other indirect effects as well.
Since tropospheric ozone tends to be highly atmospherically concentrated, its impact tends to be more local and regional. Thus, regions where its precursors - like carbon monoxide and methane - are emitted tend to have more tropospheric ozone, which is a major component of urban smog.
Tropospheric ozone isn’t visible or perceptible, but it’s certainly deadly. Exposure to it causes an estimated one million premature human deaths each year. Children, the elderly, and people with lung or cardiovascular diseases are especially susceptible to ozone-related health risks.
Tropospheric ozone impairs crop productivity and the uptake of atmospheric carbon by vegetation. It does so by impeding growth and seed production, reducing functional leaf area, and accelerating plant aging. And given the importance of plant life in just about every global ecosystem, in addition to helping the planet adapt to climate change, ozone is a major threat to life on Earth.
And ozone blunts agricultural productivity around the world. Global relative yield losses due to tropospheric ozone exposure are up to 12% for wheat, 16% for soybean, 4% for rice, and 4% for maize. As the global population grows, crop losses from forms of air pollution such as ozone could magnify the damage of climate change just as demand for food grows.
The best ways to inhibit tropospheric ozone formation generally relate to solutions to methane emissions, such as collecting landfill gas and recovering methane emissions from the fossil fuel supply chain.
Since SLCPs tend to come from similar sources, the ways to lessen their emissions tend to overlap. For instance, in agriculture, this includes better manure management, banning open-field burning of agriculture waste, and improving field aeration (particularly for rice). In waste, this includes upgrading wastewater treatment and improving waste management by separating and then treating biodegradable municipal waste and collected landfill gas. In the fossil fuel supply chain, this includes reducing leakage from gas pipelines and ensuring recovery and utilization of gas. In residential and industrial areas, this includes transitioning from traditional cooking and heating methods with cleaner biomass and pellet stoves. In transportation, this includes installing particulate filters on-road and off-road vehicles, not to mention moving away from internal combustion engines.
These fixes can provide some breathing room in the fight against the climate crisis. Scientists estimate that immediate action on SLCPs could help prevent 0.4°C of global warming and cut sea level rise by a quarter to a half. As the infamous 1.5°C threshold outlined in the Paris Agreement approaches, SLCPs can help meet this moment to limit warming in combination with long-term adaptation efforts.
Decarbonization rightfully gets the lion’s share of attention in regard to climate solutions. But SLCPs deserve plenty of focus, and reducing their prevalence would be a massive win-win for humans and the planet. A combination of ongoing and nascent SLCP solutions could alleviate a significant amount of global warming while making the world a healthier place for us and for the abundant forms of life we share this planet with.
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