For Magazine: Imagine. A national magazine for 7th-12th graders published by the
Johns Hopkins Center for Talented Youth
My perspective on global warming
Kevin E Trenberth
NCAR
PO Box 300
Boulder CO 80303
13 October 2004
Why has global sea level risen by 1¼" over the past 10 years? Why and how is the
climate changing? These scientific questions also matter for society because of the
potential impacts on storm damage and water resources, for instance, from changes in
incidence of flooding, drought and so on. Hence researching them is not only fun but also
important.
A lot of my own research has been to understand the flow of energy through the Earth’s
climate system. Earth is unique among planets because of its atmosphere and the
presence of water, water vapor, and clouds. Changes in phase of water in the
hydrological cycle play a major role in the energy cycle.
The primary driver of the climate system is the uneven distribution of incoming and
outgoing radiation on Earth. The incoming radiant energy from the sun, mainly in the
visible and ultraviolet spectra, is transformed into various other forms of energy. Some is
absorbed, raising the temperature of the air or the surface; this is called internal energy.
In land or ice, heat moves mainly by conduction, which is a slow process. For example,
major ice sheets play an important role in the climate system, but only on the scale of
millennia. In the oceans and the atmosphere, however, heat can move by convection and
advection—that is, it is carried by currents.
Some incoming energy goes into evaporation at the surface. Water molecules acquire
latent energy when they become water vapor; when those molecules condense, the
energy is returned to the atmosphere as latent heat. Some energy is converted into
potential energy, which is associated with the height of the air above the surface. Warmer
air rises and has more potential energy than cooler air. And some energy is converted
into kinetic energy, the energy associated with motion. Kinetic energy is lost by frictional
processes, which in turn heats the atmosphere or surface.
All of this energy is moved around in various ways, primarily by the atmospheric winds
and ocean currents; stored in the ocean, land, and ice components of the climate system;
and ultimately radiated back to space as infrared radiation. But not all of it makes it to
space: Greenhouse gases and clouds absorb some of it and re-radiate it up and down,
warming the planet with the greenhouse effect. Water vapor is the most important
greenhouse gas (it accounts for 60% of the current greenhouse effect for clear skies),
followed by carbon dioxide (which accounts for 26%). The latter has increased by 31% in
the past 200 years. Half of that increase has occurred since 1965, and is mostly associated
with burning of fossil fuels and other human activities.
An equilibrium climate requires a balance between the incoming and outgoing radiation
and further requires that the flows of energy are systematic. These energy flows drive the
weather systems in the atmosphere, the currents in the ocean, and fundamentally
determine the climate. The movement of heat and energy account for the rich diverse
weather phenomena we experience daily. But now that energy balance is disturbed—we
have less outgoing than incoming—so we have global warming.
As the planet heats up, the hydrological cycle speeds up. Increased heating means more
drying and increased risk of drought, but also the humidity increases and it rains harder
when it does rain, and snow may switch to rain. Hence the risk of flooding and more
intense thunderstorms and hurricanes also increases.
My research uses observations from space, weather balloons, and Earth’s surface to
estimate all the components of the hydrological and energy cycles, and to see how they
are changing over time. It seems that most of the rise in global sea level over the past 10
years is because of the expansion of the oceans as they warm, but melting of glaciers has
also contributed perhaps 20 to 35 percent.
We don’t fully understand all aspects of our climate system and how the oceans and
atmosphere interact, for instance. New technologies are enabling new measurements
from space, from specially instrumented buoys in the ocean, and from micro-weather
stations in fields or sensors on plants and organisms. The challenge is to translate the
wealth of data into knowledge and information, and build computer models to simulate
and predict the climate. Climate change matters most to the future generations. What kind
of stewards are we of spaceship Earth?