Clouds: A hot topic or are we made in the shade?
But carbon dioxide and water vapor are not the whole story. As we all know from days at the beach, clouds block much of the solar energy and reflect it back to space before it can be absorbed by the Earth, the atmosphere, or the sunbather! The more plentiful and thicker the clouds are, the cooler the Earth. At the same time, clouds also act like greenhouse gases-they block the emission of heat to space and inhibit the ability of the planet to release its absorbed solar energy. To complicate matters further, the altitude of clouds changes the amount of thermal infrared blocking. Once again, this effect is the result of the decrease in temperature with altitude-high clouds are colder, and are more effective at absorbing the surface-emitted heat in the atmosphere, while they emit very little to space because of their cold temperatures! So it turns out that clouds can either act to cool or warm the planet depending on how much of the Earth they cover, how thick they are, and how high they are. The effectiveness of clouds depends on whether they are low-altitude warm clouds made of spherical water droplets (Figure 2), or whether they are high-altitude cold clouds made up of ice crystals with a wide range of crystal shapes and sizes (Figure 3). In the late 1980s, the NASA Earth Radiation Budget Experiment (ERBE) determined for the first time that on average, clouds tend to cool the planet. The cloud reflection of sunlight back to space dominates over the clouds' greenhouse effect. In fact, the planet would on average be some 20°F hotter if we removed clouds from the atmosphere. Recently, attempts have been made to combine the ERBE satellite measurements of the radiative energy balance at the top of the atmosphere with measurements of the radiation balance at the surface. The objective of this combination is to infer the amount of radiation absorbed by the intervening atmosphere. Unexpectedly, this combination implies that the atmosphere absorbs more radiation than is theoretically predicted. Are the observations wrong or is the theory? Do we understand clouds?

Figure 2: Stratus clouds, which are mostly composed of liquid water droplets, reflect most of the incoming shortwave radiation (thin lines), but re-emit large amounts of outgoing longwave radiation (thick lines). Their overall effect is to cool the Earth.

Figure 3: Cirrus clouds, which are mostly composed of ice crystals, transmit most of the incoming shortwave radiation (thin lines), but trap some of the outgoing longwave radiation (thick lines). Their overall effect is to warm the Earth.

Given the large impact of clouds on the radiative energy balance, the critical question now becomes: What effect will clouds have on surface temperatures if global climate changes in the next century? No one knows. Clouds could act to dampen any greenhouse gas warming by increasing cloud cover, increasing thickness, or by decreasing in altitude. Conversely, clouds could act to increase warming of the planet if the opposite trends occur. In fact, the climate is so sensitive to how clouds might change, that our current best models of global climate can vary in their global warming predictions by more than a factor of three depending on how we try to model the clouds.

So why can't we model clouds? The biggest problem is that clouds are almost explosive in nature when compared to the rest of the climate system. Cumulus clouds can form in seconds to minutes, and the entire life cycle of a massive thunderstorm can be measured in hours. This thunderstorm cloud may only cover 20 to 50 miles of the Earth's surface, while our best global climate models on the world's fastest supercomputers can only track a single column of the surface and atmosphere every 50 to 200 miles.

back: Introduction
next: Surface Absorption and Reflection

Why isn't Earth as hot as an oven?

Introduction

Clouds: A hot topic or are we made in the shade?

Surface Absorption and Reflection

Atmospheric Aerosols: Fossil Fuels and Biomass Burning

From Measurements to Climate Models

Related Data Sets:

Surface Temperature

Outgoing Longwave Radiation