Clouds. They seem so simple. When they appear darker we pack
an umbrella. When they're nowhere in sight, we pack extra
sunscreen. Either way, we look to clouds for more information
than you might realize. But for all that they can tell us they
actually remain quite mysterious, especially when it
comes to how they impact the climate. The question is because
clouds are produced by the climate, how will it change in
climate impact clouds? And conversely, clouds have an
impact on our climate. So how will a change in clouds affect a
change in climate?
Welcome to Clouds 101. Like all good mysteries, this one begins
with a sophisticated scientific concept, Earth's radiation
budget. Earth's radiation budget describes the delicate balance
between the sun's radiant energy that reaches Earth, and the
radiant energy that flows from Earth back out to space. About
30% of the sun's incoming energy, essentially, the light
and heat we're familiar with is reflected back to space by
gaseous molecules in the atmosphere, tiny particles
called aerosols, land, snow and ice surfaces, and by clouds. The
remaining energy from the sun, roughly 70%, is absorbed by the
planet. Most of this absorbed energy heats up Earth's surface,
while the rest is absorbed in the atmosphere by gas molecules,
clouds, and aerosols. So heat can be both absorbed and
reflected by clouds. We'll come back to this later. Heat is also
separately emitted by earth into space in the form of thermal
infrared radiation, which is the kind of heat humans can only see
through night vision goggles. For Earth's temperature to
remain constant, the absorbed solar radiation and outgoing
thermal infrared radiation must balance one another. If the
Earth's system is changed, either through natural phenomena
like volcanic activity, or through unnatural phenomena like
humans burning fossil fuels, an imbalance in Earth's radiation
budget occurs, and as a result, the earth's temperature
eventually increases or decreases to restore an energy
balance. In recent decades, satellite and surface
measurements clearly show an energy imbalance taking place
that's been increasing. Over the past 150 years. The large rise
in carbon dioxide emissions, which accumulate in the
atmosphere, has created an enhanced greenhouse effect. This
means that energy from the sun still easily reaches Earth, but
Earth's thermal infrared radiation has a harder time
getting out into space. This has caused a decrease in how much
heat Earth sheds. Consequently, we have observed a rise in
Earth's global mean surface temperature, an increased
melting of snow and sea ice, sea level rise, and more extreme
weather events. So that brings us back to the mystery of
clouds' long term effects on climate. Here's what we know so
far. Clouds impact the radiation budget in two ways: by
reflecting solar radiation back to space, which leads to a
cooling effect on the climate, and by absorbing heat emitted
from below the clouds that would have otherwise escaped to space
if the clouds weren't present, leading to a warming effect.
Which of these effects dominates in any given location depends
upon the cloud type. High altitude clouds are typically
thinner and colder than low clouds, allowing for more solar
radiation to pass through them and reach Earth's surface. And
because they're cooler, they emit less thermal infrared
radiation to space, so they have a net warming effect on the
climate. Clouds at low altitudes, on the other hand,
are generally thicker and reflect more solar radiation
back out to space. They're also typically warmer, so they emit
more thermal infrared radiation and therefore have a net cooling
effect on the climate. We also know that when the climate
warms, Earth can respond in ways that leads to further warming.
For example, as temperatures increase, we see snowpack and
sea ice melting away in polar regions, a loss of white
surfaces that reflect the solar radiation. That means darker
colored land and oceans left behind absorb more solar
radiation, and so more heat is added to the climate system.
This cycle of more heat, more melt and more absorption of
solar radiation is called a feedback cycle. And it doesn't
end there. A feedback cycle also happens with clouds. Climate
models predict a decrease in low altitude cloud coverage over the
globe as the climate warms, since low clouds are the highly
reflective type, a decrease in low cloud coverage means more
heat will be added to the Earth's system, leading to
further warming. And clouds impact the climate in another
way too: Through the water cycle producing rain and snowfall.
Water at Earth's surface evaporates, providing the
atmosphere with a supply of water vapor. Depending on the
air temperature and atmospheric pressure, the air can only hold
so much water vapor until it becomes saturated. When that air
saturated with water vapor cools, the water vapor turns
back into liquid water droplets and forms clouds.
When these droplets or ice crystals accumulate, that is
what we call a cloud. When the droplets or ice crystals within
the cloud grow to be large enough, they eventually fall to
the ground or ocean as rain, snow or hail. This brings us
back to the mystery at hand. Because clouds both reflect and
absorb energy from the sun impacting both ends of the
radiation balance and play a massive role in the water cycle,
any changes in clouds will result in a change in our
climate. But clouds are also produced by our climate, so any
change in climate will result in a change in clouds. As you
probably now realize the relationship between clouds and
the climate is incredibly complex, and NASA is on a
mission to understand it. Using NASA's Earth Observing fleet of
satellites like Calipso and instruments like Ceres and
MODIS, scientists have been collecting vital data on clouds
to be able to precisely model their behavior. A key ingredient
to unraveling the mystery of clouds lies in the collection of
global accurate multi decadal climate data records of cloud
properties and their influence on Earth's radiation budget.
Once we can accurately and fully understand the physics of clouds
through observations, that data can then be used to help improve
climate and weather models so we can better prepare for the future.