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Climate Change

Do we know what it means to engineer the climate?

Article: Fasullo, J.; Tilmes, S.; Richter, J.; Kravitz, B.; MacMartin, D.; Mills, M.; Simpson, I. (2018). Persistent polar ocean warming in a strategically geoengineered climate. In: Nature Geoscience. DOI: 10.1038/s41561-018-0249-7.

At this point, it’s undeniable that the climate is changing rapidly. There is indisputable proof that global temperatures have increased at least 1°C above preindustrial levels, and the subsequent effects are expected to intensify as temperatures continue to rise.

One consequence of this warming is sea level rise via glacial melt and expansion of water as it warms. Higher temperatures in the Arctic are reducing the amount of ice, thereby reducing how much sunlight reflects back to space (also known as albedo), leading to even warmer conditions near the poles. This is significant because temperature differences between the poles and equator drive the redistribution of heat through the atmosphere and ocean, which helps maintain the various climates familiar to life on Earth.

Most cities do not have the infrastructure to effectively handle heavier and more persistent rain enabled by a warmer atmosphere, nor have they been designed to prevent massive fires from spreading uncontrollably during abnormally dry conditions. More intense hurricanes are becoming a frequent occurrence, wiping out entire coastal communities. Cold spells and heat waves are not only an inconvenience to human life, but can devastate wildlife habitats. For example, increased ocean temperatures can alter communities of oxygen-producing, carbon-removing phytoplankton, stressing marine ecosystems by depriving them of oxygen and food.

The far-reaching effects of climate change have implications for the livelihood of all of Earth’s inhabitants; however, most of them (including people) have no direct control over most of the carbon dioxide (CO2) emissions driving it. The diversity of life on Earth, our food resources, the comforts of a habitable environment – these are just some examples of what’s at stake under climate change largely driven by those with the most power and money in society.

Geoengineering our way to safety… maybe

Climate concerns have led policymakers and scientists to ponder our options for taking action. While the latest report by the Intergovernmental Panel on Climate Change (IPCC) stated that we must reduce our global emissions to 55% by 2030 and to ‘net-zero’ by 2050 to limit warming to 1.5°C, researchers are also considering the potential of geoengineering – the attempt to manually change the climate system to reduce the impacts of climate change. Geoengineering methods are being considered in addition to mitigation (reduction of emissions) and adaptation (reduction of vulnerability to climate risks). One popular option is carbon management – that is, removing excess carbon dioxide from the atmosphere.

Another possibility is solar radiation management (SRM) – the injection of aerosols (clouds of tiny particles) into our stratosphere (upper atmosphere) that reflect the sun’s rays back into space, increasing its albedo. The idea is to counteract the increased heat retained by greenhouse gases like CO2 by deflecting more sunlight.

Geoengineering strategies like SRM have been confined to scientific study due to their potential risks, but the dangers of climate change are becoming urgent. If we can quickly reduce climate change with SRM, why haven’t we already done it? The main problem with this idea is that the climate is an unpredictable system. We don’t know how it will respond to being manipulated, and it could just make things a lot worse. We don’t know nearly enough to reliably implement SRM right now, but scientists are trying to change that.

Manipulating the climate in computer models

This study, led by Dr. John Fasullo at the National Center for Atmospheric Research (NCAR), addresses the potential impacts of SRM on our climate system by using a state-of-the-art climate model to simulate the motion in the ocean and atmosphere (known as the Community Earth System Model). They compare business-as-usual emissions conditions (CO2 emissions stay as high as they are now) to the same conditions if aerosols were injected into the stratosphere at 15° and 30° latitude in both hemispheres. The hope is that by strategically injecting aerosols at specific latitudes from the year 2020 until the end of the century, the dimmed sunlight will regulate not only the global average temperature, but the north-south temperature differences that drive wind, ocean currents, and precipitation. All of these processes can push back onto the manipulated climate, changing it in unexpected ways.

Solar dimming revs up ocean circulation

Fig. 1: Air-sea fluxes of heat (a) and freshwater (b). In (a), red indicates heat flux from ocean to atmosphere; blue is opposite. In (b), brown indicates freshwater flux from ocean to atmosphere; green is opposite. Arrows indicate near-surface winds (Reprinted with permission from Nature Springer: Fasullo et al. 2018).

The simulation results show that, as expected, the SRM successfully reduced the global average surface temperature, in turn reducing global precipitation by lowering the heat needed for evaporation.

But there’s a catch.  One of the most prominent results of this study shows abnormally high heat content and heat transfer from the ocean to the atmosphere in the north Atlantic (above 45°N), an important region for global ocean circulation (Fig. 1).

More specifically, SRM changed the Atlantic meridional overturning circulation (AMOC) – a system of ocean currents in the Atlantic that redistributes ocean heat on long time scales, significantly impacting our climate. The high heat content in the north Atlantic indicates that SRM would speed up the AMOC (increasing ocean heat transport towards the poles) – the reverse of what global warming is expected to do.

Changes in precipitation and evaporation are the primary drivers of this phenomenon. Under this simulation, the subtropics (between about 23.5°N and 50°N) experience less evaporation due to reduced heating from the sun, while the north Atlantic experiences less rainfall, making it saltier. Wind increases, revving up the ocean’s surface currents and allowing more warm water to enter the north Atlantic from the south. Warmer water in this region causes more evaporation, taking fresh water and heat out of the surface and leaving behind denser, saltier water which quickly sinks to the bottom of the ocean (Fig. 2). The whole  process is like AMOC on steroids – SRM causes much faster heat transport to the poles than what occurs today. This forces more heat into the north Atlantic, and then into the deep ocean.

Fig. 2: A north-south transect of temperature (c) and salinity (d). Both show a collection of warm, salty water that sinks to the deep ocean in the north Atlantic (Reprinted with permission from Nature Springer: Fasullo et al. 2018).

So… is SRM good?

These results have broad implications for the potential of SRM and climate models. For starters, increased heat in the north Atlantic could enhance glacial and sea ice melt, counteracting some of the effects of SRM. Enhanced ocean heat content also has the tendency to expand ocean water and increase sea level. Both ice melt and water expansion are not promising in terms of SRM reducing sea level rise. Also, changes to AMOC and its relationship to other forms of climate variability in the region are still poorly understood.

In short, we don’t have a good idea of what revving up the AMOC will really do to our climate, given that climate models have yet to resolve all the significant components of the system. The truth is, a lot more work needs to be done to assess the viability of SRM and geoengineering strategies in general before we can even consider their implementation. Toying with the climate may have unintentional consequences, some of which could be catastrophic and irreversible. It’s important that the public is informed, not only about the potential of geoengineering, but its considerable risks as well.

Although most of us don’t belong to those powerful elite who drove us into our burgeoning climate catastrophe, we have the right to decide how we want to get out of it.

To learn more about SRM and its political implications, I recommend this recent article.

I’m a 4th year PhD student at the University of Rhode Island Graduate School of Oceanography. I use models to study small-scale turbulence at the air-sea interface induced by airflow over surface gravity waves to understand how wind-wave interactions impact wind stress, or air-sea momentum transfer. Wind stress encompasses a range of scales, producing ripples to planetary waves, driving coastal currents and ocean circulation, and modulating weather and climate. In the future, I hope to learn more about the role wind stress plays in the variability of the ocean and atmosphere. Also, I love to write.

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