Facts of the Matter
Summer hurricanes and tropical storms transfer great amounts of heat from the tropics to the mid-latitudes.
Climate change is not an exact science
WITH there being little doubt that the Earth is warming, whatever the extent of our culpability in it, and with the warm days of autumn approaching, it seems a good time to think about heat budgets.
If heat gain exceeds heat loss there is an increase in the amount stored, which manifests as an increase in temperature. If loss exceeds gain the temperature decreases. It's a basic principle of physics as well as economics.
Methods of heat transfer are well understood. There are "sensible" heat transfer processes of conduction, convection, and radiation, and latent processes of melting and freezing, evaporation and condensation.
For a large, complex system such as Earth, the processes are difficult to track. It is difficult enough to measure Earth's overall temperature.
Measuring its heat budget presents even greater difficulties.
The income portion of Earth's heat budget is the sun's energy. It is reflected back into space or absorbed in varying amounts by atmosphere, clouds, water, and land. The heating is uneven at best due to seasonal changes in the sun's local angle above the horizon, changing day/night lengths, vegetation and cloud cover.
Earth eventually radiates heat back into space in the form of longwave or infrared radiation. The outgoing heat approximates the amount incoming, but it is thwarted to varying degrees by clouds, pollution, and greenhouse gases that absorb outgoing longwave radiation and radiate it back toward the surface to act as a blanket.
The most notorious of these is carbon dioxide, but water vapor, which is typically the third most abundant gas in the atmosphere after nitrogen and oxygen, and methane produced by decaying organic material, animals, and various outgassing processes from Earth's surface are major factors.
The sun's output fluctuates more than previously thought, adding to the complexity by varying the input by a few percentage points. Meanwhile Earth's weather systems vary in number and intensity from one year to the next. Global surface ocean currents are driven by global air circulation patterns and deep circulation is driven by temperature gradients in surface ocean water.
In any given year, the incoming and outgoing amounts may not quite add up. There are deficits and excesses in both incoming and outgoing radiation that cause Earth's average temperature to rise and fall over time scales ranging from years to millennia.
As expected, Earth receives more heat at the equator and progressively less poleward in both North and South, with the least received at the poles.
The tropics lose less heat than they absorb from the sun while the poles lose more than they receive. The gains and losses are nearly equal around 40 degrees North and South latitude. It is apparent that heat is transferred relentlessly from the equator poleward, but the rate is not steady
It is difficult to measure the details of Earth's heat budget. Summer hurricanes and tropical storms transfer tremendous amounts of heat from the tropics to mid-latitudes, while winter cyclonic storms transfer heat from mid-latitudes to higher Arctic latitudes. These storms vary in strength from year to year, and the intensity increases as the amount of available energy increases.
Over the short term, which by Earth standards may be decades, centuries, or even millennia, excess heat can accumulate in the tropics, or be moved in excess poleward.
Virtually unknown until the 1970s, such seemingly local effects as El Nino are now known to be part of larger interacting global couplings between atmosphere and ocean and between different parts of the globe.
El Nino is intimately linked with the Asian monsoon and with flooding and droughts in Australia, North America, Africa and Europe.
Temperatures in Saharan Africa and in the tropical Atlantic influence hurricane formation and intensity while desert temperatures in Mexico influence formation of Pacific hurricanes.
The speed and intensity of ocean currents such as the Gulf Stream affect temperatures of Arctic and Antarctic waters. To the south of Greenland air and water temperature and ocean salinity affect the "great conveyor belt" of deep ocean circulation, which may be the single most significant source of heat transfer on the planet.
The greatest control on polar ocean salinity is melting of polar and continental ice, such as the great ice sheets that cover Greenland and Antarctica.
The global climate systems make adjustments in various ways, most of which are little understood, many of which are under intensive study as we try to measure the amount of global warming and forecast both long and short term effects and the extent of our influence on it.
Over 3 billion years of its existence Earth has developed many checks and balances on extremes.
As temperature increases, more water evaporates, causing higher humidity and more clouds. Clouds reflect solar radiation, but humidity increases absorption of outgoing longwave radiation and also reflect heat back to the surface. Which effect is dominant and under what conditions? No one knows.
Carbon dioxide and water are used by plants during photosynthesis. Higher concentrations of carbon dioxide, humidity and more clouds create more rain to allow more green plants that have a cooling effect.
But at the same time, more plants allows a larger population of animals, which increases methane concentration, and the decay of plants and animals increases both carbon dioxide and methane.
For at least the past 550 to 600 million years, Earth's climate has fought a continuing battle between temperature and the balance between oxygen and carbon dioxide.
We know that there have been several cycles of extreme imbalance when temperatures have been either much lower or much higher than today. We are just coming out of a 2 million year ice age that saw episodes of growth and decline of ice, which at times covered North America with a two-mile thick ice cap as far south as southern New York with corresponding changes in worldwide sea level of hundreds of feet.
There are many possible causes, among them drifting continents, comet or asteroid impacts, volcanic activity, periodic changes in Earth's orbital parameters, or others we don't even know about yet.
It is far from an exact science. There are too many variables, the planet is too large, evidence from the past is too scant, current data is too sparse, and our computing power is orders of magnitude too weak to run reliable numerical models.
The truth is, we just don't know much about climate change. We don't know what to expect, in what time frame, how much we are responsible, what to do about it in any case
We could do nothing and wait to see what happens. We could do the wrong thing and make it worse. We could do the right thing, but not know it because a two-mile asteroid smashes into the planet or volcanic activity increases, or the sun decides capriciously to change its output of radiation just a little one way or the other.
That is why we keep studying, keep collecting more data about the past and present, and keep trying to understand.
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at firstname.lastname@example.org