Can global warming be reversed?
Clearly, neither ice age, nor asteroid impact or the magnitude and frequency of volcanic eruptions
of a scale that would cancel the effects of global warming are imminent
Quamrul Haider
Based on a range of plausible emission scenarios, studies by different research groups, including the Intergovernmental Panel on Climate Change (IPCC), are predicting that global temperature is likely to rise more than two degrees Celsius by the end of this century. However, with the withdrawal of the United States from the 2016 Paris Agreement on Climate Change, global warming is no longer a matter of the next 100 years. Furthermore, after the head-in-the-sand Donald Trump gave US companies carte blanche to pollute the atmosphere, climate scientists are now more concerned about how greenhouse gas emissions will play out over many millennia.
In fact, a large group of leading climate scientists, including very influential ones from MIT, University of Bern, Oregon State University, Oxford University and the IPCC, writing in a high impact factor academic journal (Nature Climate Change February 2016), made disquieting predictions about global temperature and sea level ten thousand years from now. They wrote that the Earth could ultimately be an astonishing seven degrees Celsius warmer on average and feature seas 52 meters higher than they are now. How life on such a planet will fare is impossible to predict. Nonetheless, adapting to a warmer Earth with coastal cities and low-lying coastal nations completely submerged in water will be a long and painful process. Many species wont be able to make it through. Human beings are not guaranteed to survive either.
These predictions raise several questions. Is there a way out of this conundrum? Can global warming be reversed or stabilized? What would happen to the climate if we stopped emitting greenhouse gases today, right now?
There is no simple answer to the first two questions. As for the third question, putting the brakes on the emission of greenhouse gases will not reverse or stabilize global warming soon enough to prevent our planet from becoming uninhabitable. Thats because hundreds of billions of tonnes of heat-trapping, long-living carbon dioxide poured by us into the atmosphere since the Industrial Revolution will continue to affect the climate for several thousands of years before removal by natural processes, such as chemical weathering, rock formation and absorption by oceans.
Are there other natural processes that could reverse global warming in a shorter period of time? Yes, there are. One of the natural phenomena that lowered the global temperature in the past was glaciation during an Ice Age. At the height of the last ice age some 25,000 years ago, average global temperature may have been as much as 10 degrees Celsius colder than today. That being said, while Homo sapiens evolved during the last ice age, many megafauna became extinct due to harsh climate conditions. In addition, scientists claim that a strip of land on the southern coast of Africa was the only place that was habitable during that time.
Human effects on the climate notwithstanding, the cycle of ice ages will continue as long as the Earth rotates about its axis and orbits around the Sun. The hot periods will eventually come to an end and the ice sheets will descend again. Can we, therefore, hope for the onset of the next ice age before long to cool our planet?
The beginning of ice ages and the glacial cycles within them depend on the complex interplay of three celestial phenomena involving the Earth. They are periodic elongation of the Earths orbit due to gravitational tugs from Jupiter and Saturn, precession of its rotational axis and variation in the axial tilt. The start and end of ice ages are also influenced by many geological and climatic factors. Among them are changes in topography and plate tectonic motions, the hydrosphere, the biosphere and volcanic eruptions. Moreover, combination of low concentration of greenhouse gases in the atmosphere and lower sunlight at latitudes north of the Arctic Circle, where snow surviving through the summer leads to ice sheets, were precursors to the dawn of the past ice ages.
The stage for the next ice age will be set about fifteen to twenty thousand years from now, when the rhythmic wiggles in the Earths orbit, and precession and tilt of the axis are expected to resonate. Nevertheless, debate persists about how our anthropogenically influenced global warming will disrupt the natural cycles of ice age. A recent study concludes that because of the greenhouse gases in the atmosphere, the deep freeze will not occur for another 100,000 years. Thats a long time. Will the global temperature remain within tolerable limit for so many years? The answer is anyones guess.
A series of massive volcanic eruptions is the other natural phenomenon that could cool the Earth. When volcanoes erupt, they spew huge quantities of mostly sulfate particles into the air. Larger particles fall out of the air quickly, while the smaller ones travel vast distances, including to the stratosphere ‒ the layer of the atmosphere extending to approximately 50 km above the Earths surface ‒ and create a shield that greatly reduces solar heating of the Earth. Because these particles can stay in the stratosphere for several years, there will be significant cooling worldwide.
By how much and for how long an eruption can cool the Earth would of course depend on the amount of material it ejects and how high it reaches. Additionally, because of circulation patterns in the atmosphere, eruptions in the tropics would have cooling effect in both hemispheres while eruptions at mid or high latitudes would cool only the hemisphere they are within.
Indeed, in 1784, Benjamin Franklin, one of the founding fathers of the United States and a renowned polymath, established the scientific connection between the abnormally cold summer of 1783, both in Europe and in the US, with enormous eruptions of a chain of volcanoes in Iceland that lasted for eight months. The atmospheric effects of volcanic eruptions were further confirmed by the 1991 eruption of Mount Pinatubo in the Philippines. Pinatubos eruption cloud cooled the Northern Hemisphere by about 0.6 degrees Celsius during 1992 and 1993.
There are some obvious tradeoffs of volcanic cooling, though. A major volcanic eruption represents a real risk to life and property. It can also cause earthquakes, flash floods, fires and mudslides, as well as temporarily disrupt air traffic. It can change global rainfall patterns, too.
We cannot rule out the possibility of an astronomical event, such as an asteroid impact, to negate global warming. According to the US National Center for Atmospheric Research in Boulder, Colorado, the soot and dust that would shroud the Earths atmosphere after the impact of a medium-sized asteroid would reduce the amount of sunlight hitting the surface sufficiently enough to lower the global temperature by 5-8 degrees Celsius. Thats what happened 65 million years ago, and the climate did cool appreciably. As a somber reminder, the aftereffects of the asteroid impact also wiped out the entire population of dinosaurs from the face of the Earth.
Astronomers almost unanimously agree that no asteroid will hit the Earth in our lifetime. But theres no reason to think it wont happen in the future. After all, more than 90 percent of the biggest asteroids are lurking near the Earth. As recent as December 2017, a 5-km wide asteroid, known as 3200 Phaethon, hurtled towards the Earth and zoomed past at a distance of 10-million kilometers, which is a stones throw on the astronomical scale. It was large enough to unleash a mini ice age had it struck the Earth.
Clearly, neither ice age, nor asteroid impact or the magnitude and frequency of volcanic eruptions of a scale that would cancel the effects of global warming are imminent. Hence, unless we find other ways to lower the global thermostat, we will be handing over a planet to our future generations which will be close to unlivable.
The writer is a Professor of Physics at Fordham University, New York
