Why Reflectivity?
Brian Smoliak and Michel Gelobter
November 8, 2022

Once every March and September, sunlight bathes each location on our planet for 12 hours. These biannual equinoxes also mark the transition between seasons, dividing warm and cold in the extratropics and wet and dry in the tropics. Sunlight is the primary energy input to the Earth system. It warms Earth’s surface and provides a key ingredient necessary for life. Earth’s energy budget is balanced by heat emitted by the surface and atmosphere back into space. Scientists have estimated that globally about 240 Watts per square meter continuously enter and leave the Earth system.

Earth’s energy budget is balanced by incoming sunlight and outgoing heat. Image credit: NASA.

In the age of climate change, the flows of energy through the Earth system have changed slightly so that 240 Watts per square meter are coming in and 239 Watts per square meter are going out. This 1 Watt per square meter imbalance is due to increasing concentrations of atmospheric greenhouse gasses, which trap some of the outgoing energy like a clogged drain. As long as human activities continue to emit such large amounts of greenhouse gasses, Earth’s energy imbalance will continue to increase.

LED light bulb. Image credit: Frederico Bottos

1 Watt per square meter might not seem like much, just enough to power a small LED bulb. However, it adds up when accumulated continually over time and eventually we notice its effects. Our knowledge of physics suggests that this surplus energy should melt ice, evaporate moisture, and warm the water, land, and atmosphere. Sure enough, our understanding is confirmed by observations of melting ice sheets, glaciers, and sea ice, an increased hydrologic cycle over evaporation and precipitation, and warming oceans, land, and atmosphere.

The warming we’ve observed in our lives on Earth’s surface is a small fraction of the overall buildup of energy in the climate system. Over 90% of the energy is stored as heat in the ocean. Melting ice is the next largest contributor to Earth’s energy imbalance, followed by land warming, and finally atmospheric warming. These changes pose enormous challenges for aquatic and terrestrial ecosystems, human activities like agriculture, transportation, and commerce, and perhaps most importantly, human health and well-being.

Reducing Earth’s Energy Imbalance

If we accept that Earth’s energy imbalance is a problem, where can we look for solutions? At a fundamental level, we should look to factors that influence either component of Earth’s energy budget: incoming sunlight and outgoing heat. To date, climate action has tended to focus on increasing outgoing heat by reducing concentrations of heat-trapping greenhouse gasses like carbon dioxide and methane. However, emissions and atmospheric concentrations of greenhouse gasses continue to increase in spite of thirty years of coordinated international efforts.

Carbon dioxide concentration pathways under different emission scenarios: increasing emissions, stabilized emissions, reduced emissions, and stopping emissions (i.e. net zero). Notice how CO2 concentrations continue to increase unless emissions reach net zero.          Image credit: NASA.

Reflective Earth takes a broader perspective by working on reflectivity. We focus on actions that decrease incoming sunlight and ways to increase outgoing heat beyond emissions reductions. Reducing sunlight by less than 0.5% would compensate for Earth’s energy imbalance and provide a longer runway for society to restore atmospheric greenhouse gas concentrations to pre-industrial levels. Most surfaces are efficient emitters of heat, however there are opportunities to increase Earth’s surface emissivity, which would produce a cooling effect by boosting outgoing heat.

In the long run, Earth’s climate responds directly to concentrations of greenhouse gasses. While society does the slow, difficult work of decarbonization, Reflective Earth is focused on more rapid ways to slow global warming.

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