This article has been edited by atmospheric physicist Thomas Choularton
Historically, Earth has been subject to a rhythm of climatic transformations. Geological studies have been able to piece together significant paleoclimate phases to a timeline spanning billions of years. Today, a unique challenge dawns on present and future scientists to slow down the clock of a warming climate, accelerating at an unprecedented pace.
When zooming out on the Earth’s timeline to view its ancient entirety, we see that the planet has existed in a flux of climatic changes. At present, we are living in what is believed to be our fifth ice age, meaning three million years ago Earth had no polar ice sheets. The last 11,000 years of history—considered the Holocene Epoch—is an interglacial 'warm phase' of the current ice age.
Zooming in now to this 11,000-year segment of time, minor climate fluctuations can still be observed. It has been hypothesised that between 900 and 1300 AD, temperatures were about one degree warmer in Europe than they are today. Yet, the northern hemisphere experienced a “mini ice age” between 1650 to 1715, which coincided with a Grand Solar Minimum, a reduction in solar irradiance—the energy Earth receives from the sun. Earth and space scientists from NASA explain that a Grand Solar Minimum today could offset warming from three years' worth of CO2 growth, yet this would be insignificant in reducing long term warming.
These scientists also argue that the cause of major climatic phases—going back to the ice ages and glacial/interglacial periods—has largely been driven by the processes involved in Earth's orbit—these are known as the Milankovitch cycles.
Serbian astronomer Milutin Milankovitch realised in the 1920s that the combination of long-term changes in the Earth’s orbit from circular to elliptical (eccentricity), its axis of tilt varying from 22.1 and 24.5 degrees (obliquity) and its wobble on its axis (precession) together influence the seasonal patterns that life on Earth depends on. Both obliquity and precession take tens of thousands of years to transform from one extreme to the other, while the eccentricity cycle spans 100,000 years.
As the Milankovitch cycle determines where solar energy is received, its control on the length and intensity of summer holds supreme power over all ecological demographics. Over periods where summers are short and mild, ice and glaciers have succeeded to establish at the Earth’s poles and beyond. The Earth’s orbit is the clockwork to all flora and fauna. Species either evolve to occupy the environmental state—growing, migrating and reproducing to the rhythm of the seasons—or vanish from the ecosystem entirely.
While the Earth's circuit is responsible for the distribution of solar energy, processes that occur on Earth’s surface influence how that solar energy redistributes itself around the planet. Ocean and air currents are significant heat dispersal operators that actively mitigate the temperature inequality between polar and equatorial regions. Continental drift has redrawn the paths of ocean currents that carry the equator's heat to the poles, and the development of mountains and topography has shifted the flow of air, forcing it to duck and dive over landforms.
The Earth balances incoming solar radiation (shortwave radiation) with outgoing infrared radiation (longwave radiation), creating the perfect conditions to support life. Energy in, energy out.
Shortwave radiation is either absorbed by Earth’s surfaces or reflected, commonly by clouds, glaciers and ice sheets—this reflectivity is known as 'albedo'. In exchange for this heat income, longwave radiation travels from Earth’s surface, through the layers of the atmosphere and is released into outer space. This journey has varying degrees of success, as outgoing radiation is confronted by major obstructions—the notorious greenhouse gases carbon dioxide and methane. The molecules of these gases vibrate in ways that oxygen and nitrogen cannot, allowing longwave photons to be absorbed and re-radiated in all directions. But before longwave photons are sent on their way once again, heat energy is shared to surrounding particles in the atmosphere.
When longwave radiation is obstructed in this way, heat energy may be reradiated back towards Earth’s surface, maintaining surface warmth. This is a natural process that has regulated a snug environment on Earth for a long time. Only in recent history has this process been enhanced by the excessive quantities of obstructive gases in the atmosphere, which is a result of our industrial society. Today, carbon dioxide and methane are absorbing and redirecting longwave radiation to unnatural extents. While methane molecules are far more dangerous in their efficiency at redirecting thermal radiation, carbon dioxide molecules linger in the atmosphere for a much longer period of time and therefore have accumulated in our atmosphere to monumental quantities since pre-industrial times.
Under pressure (do do-do do do)
Look up from wherever you are reading this. Observe what you see. You are looking at a knitted mass of gaseous elements; a mosaic of particles—only, you can’t see them. The atmosphere is full of chemical elements, swirling above our heads: water vapour, carbon, argon and nitrogen, with the latter making up 80 percent of our air.
Earth's inhabitants essentially live in an invisible shell of toasty, chemically dense jelly, if you will—jelly that contains the nutrients our cells slurp up to produce the chemical energy required for body function. The atmosphere is not only crucial for respiration but also for maintaining the temperatures that us humans and our fellow Earth-residing organisms require for survival.
This atmospheric 'jelly' has a density gradient with heat and gas concentrated at the surface and diluted at higher altitudes, which means that the temperature also decreases with altitude. Close to the surface, warm gas molecules are densely packed together, and their weight increases their gravitational pull to the surface. Temperature and air pressure have a co-dependent relationship; with less gas density at higher altitudes, molecules have the room to lose energy and cool off. However, air pressure might not be uniform across a given altitude in the sky. When cold and warm air masses sit shoulder to shoulder, air tends to move horizontally—which we know as wind.
Warmer air masses are essentially a thicker jelly. In tropical regions, for example, it takes a greater climb in altitude to reach really low air pressure where temperature is freezing than, for example, the mid-latitudes. There’s a particular temperature in the atmosphere where longwave energy can be released into space; the altitude of this temperature is called the ‘Skin Altitude’. Because air temperatures differ across Earth, this Skin Altitude is higher in the tropics—where the air is thick and dense—than at the poles, where air pressure drops quickly with height. As the atmosphere is warming worldwide, the Skin Altitude is regularly getting pushed further from Earth's surface at the tropics, the poles and everywhere in between.
There are dramatic consequences that come with having a higher Skin Altitude. Mainly, it means that longwave energy has to travel farther through the atmosphere before it can emit to outer space, which provides more opportunity for it to be absorbed by the pesky greenhouse gases that we release. The obstruction of longwave energy by gases threatens to disturb the equilibrium of incoming and outgoing energy (remember, “energy in, energy out”). The Earth, however, is committed to maintaining this energy budget. It compensates for the obstruction of longwave energy by warming the surface. Now, additional longwave energy can be exerted and the balance of longwave and shortwave energy can continue.
In short, the Skin Altitude rises, the ground warms, the air warms, the Skin Altitude rises, and the pattern repeats. It’s a perpetual cycle that is amplified by the excess quantity of greenhouse gases and the over-absorption of longwave radiation in the atmosphere, which human activity has exacerbated.
“Water” we going to do about this weather?
Water vapour is a complex greenhouse gas. The fifth IPCC assessment report was consistent with general conclusions among science groups that the role of clouds in future climate change is vastly uncertain.
One on hand, water vapour is a major obstruction to the Earth’s longwave radiation. The lopsided characteristic of H2O molecules enables them to absorb lots of frequencies of infrared light. High cirrus clouds catch longwave radiation at altitudes where it would otherwise emit to space. Yet, on the other hand, water serves to mitigate warming as large portions of solar heat are reflected by ice, low cloud and ocean surfaces, or stored away in the depths of the ocean. On a micro-scale, though the heat produced from condensation warms the air, water has a talent for cooling the air again with a familiar phenomenon—one that always seems to occur on the days you leave your umbrella at home—rain.
Rain no longer retains a romantic ambience when it thunders down in a raging tantrum, bowling down trees and upturning houses. The Hadley cell has significant involvement in the catastrophic rainfall experienced in equatorial regions. The Hadley cell is a circulation of air currents that acts to redistribute from warmer to colder areas of the globe. In this process, high trade winds blow from the equator to the poles before descending and returning to the equator closely above the ocean surface. The circuit can repeat itself after warm equatorial air rises from the surface. During this ascent, water vapour is carried in the air to higher altitudes, where it eventually condenses into heavy clouds.
Heavy rain is inevitable in equatorial regions, as warmer air has a larger capacity to hold water vapour. Rising maximum air temperatures will expand this capacity, threatening higher intensity storms in the future. Water vapour helps intensify hurricanes and cyclones like adding gasoline to a fire. Condensation of atmospheric moisture exerts heat energy which boosts the energy supply of upper winds. These spinning vortexes of cloud and wind that terrorise communities are gaining the strength to become more colossal and more devastating.
Challenges that lie ahead
For much of Earth's history, the atmosphere has maintained the temperatures necessary for our planet's inhabitants to survive and succeed. The genetic tools that have been gifted to us from our ancestors are fundamental to our ongoing survival. Our DNA programmed us to have legs to move, hands to forage for resources and an immune system to fight viruses. The environment that our ancestors occupied is fundamental to how we came to be.
When the environment changes over time, species adapt their structures and behaviours. We call this evolution. But when our environment changes rapidly, it forces individuals to acclimatise—to develop new strategies to live. It takes real strength to overcome sudden challenges. For most species, the energy expended to confront change is life-threatening. That old family recipe, passed down from generation to generation, must now be abandoned. Adapting to the fast pace of change is achievable for humans due to the technologies we have developed, but less so for some who we share our planet with. For many species, evolving and altering structural DNA is a long and complex process that cannot occur at the pace climate change demands.
The disturbances that human activity has caused to the climate's natural cycles is giving rise to a contemporary ecosystem that favours those species that are flexible to changes in air, water, vegetation and landscape—as well as the indirect effects that follow. While the beloved giant panda is growing hungry as its habitat and food sources disappear, the malaria mosquito thrives in the heatwaves of Botswana. For countries around the world, the dangers facing native biodiversity are a distressing reality. Though our society can feel divided by national and cultural differences, the world is united in dealing with this ecological crisis.
We don’t see the daily clouds of greenhouse gases that we emit or their interception of longwave radiation. But we are meddling with our own habitat in order to pursue a system that prioritises economic progress. We are meddling with the habitat of species who receive no benefits from this system. Above all, this system cannot be sustained. It is unsurprising, then, that our fellow Earth-dwellers—who hop and swim and fly beside us—are battling the toll of climate change as greatly as we are. The difference is that we can do something about it.