Why the Aral Sea Became Shallow

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From "Defusing the Global Warming Time Bomb"

A climate forcing is a mechanism that alters the global energy balance. A forcing can be natural -fluctuations in the earth's orbit, for example - or human-made, such as aerosols and greenhouse gases. Human-made climate forcings now dominate natural forcings. Carbon dioxide is the largest forcing, but air pollutants (black carbon, ozone, methane) together are comparable. (Aerosol effects are not known accurately).

It is well known that human-made greenhouse gases, especially carbon dioxide, have been accumulating in the atmosphere. These gases are a climate "forcing", a perturbation imposed on the energy budget of the planet. Like a blanket, they absorb infrared (heat) radiation that would otherwise escape from the earth's surface and atmosphere to space.

How much will climate change in coming decades? What will be the practical consequences? What, if anything, should we do about it?

Objective analysis of global warming requires knowledge of three issues: the sensitivity of the climate system to forcings, the forcings that humans are introducing, and the time required for climate to respond. All these issues can be studied with global climate models, which are numerical simulations on computers. But our most accurate knowledge about climate sensitivity, at least so far, is based on empirical data from the earth's history.

The lessons of history

Over the past few million years the earth's climate has swung repeatedly between ice ages and warm interglacial periods. A 400.000-year record of temperature is preserved in the Antarctic ice sheet, which, except for coastal fringes, escaped melting even in the warmest interglacial periods. This record suggests that the present interglacial period (the Holocene). now about 12.000 years old. is already long of tooth'.

The natural millennial climate swings are associated with slow variations of the earth's orbit induced by the gravity of other planets, mainly Jupiter and Saturn (because they are so heavy) and Venus (because it comes so close). These perturbations hardly affect the annual mean solar energy striking the earth, but they alter the geographical and seasonal distribution of incoming solar energy, or insolation, as much as 20 percent. The insolation changes, over long periods, affect the building and melting of ice sheets.

Insolation and climate changes also affect uptake and release of carbon dioxide and methane by plants, soil and the ocean. Climotologists are still developing a quantitative understanding of the mechanisms by which the ocean and land release carbon dioxide and methane as the earth warms, but the paleoclimate data are already a gold mine of information. The most critical insight that the ice age climate swings provide is an empirical measure of climate sensitivity.

The composition of the ice age atmosphere is known precisely from air bubbles trapped as the Antarctic and Greenland ice sheets and numerous mountain glaciers built up from annual snowfall. Furthermore, the geographical distributions of the ice sheets, vegetation cover and coastlines during the ice age are well mapped. From these data we know

¹ to be long in the tooth - быть не первой молодости

that the change of climate forcing between the ice age and today was about 6.5 watts per square meter.

Antarctic ice has preserved a 400,000-y ear record of temperature and of levels of carbon dioxide and methane in the atmosphere. Scientists study gases trapped in air bubbles in the ice - generally using ice cores extracted from the ice sheet and transported to a laboratory. The historical record provides us with two critical measures: Comparison of the current interglacial period (the Holocene) with the most recent ice age (20.000 years ago) gives an accurate measure of climate sensitivity to forcings. The temperature in the previous interglacial period, when sea level was several meters higher than today, defines an estimate of the warming that today's civilization would consider to be dangerous anthropogenic interference with climate.

Climate-Forcing Agents Today

The largest change of climate forcings in recent centuries is caused by human-made greenhouse gases. Greenhouse gases in the atmosphere absorb heat radiation rather than letting it escape into space. In effect, they make the proverbial blanket thicker, returning more heat toward the ground rather than letting it escape to space. The earth then is radiating less energy to space than it absorbs from the sun. This temporary planetary energy imbalance results in the earth's gradual warming.

Because of the large capacity of the oceans to absorb heat, it takes the earth about a century to approach this balance - that is, for it to once again receive the same amount of energy from the sun that it radiates from space. And of course the balance is reset at a higher temperature. In the meantime, before it achieves this equilibrium, more forcings may be added.

The single most important human-made greenhouse gas is carbon dioxide, which comes mainly from burning fossil fuels (coal, oil and gas). Yet the combined effect of the other human-made gases is comparable. These other gases, especially tropospheric ozone and its precursors, including methane, are ingredients in smog that damage human health and agricultural productivity.

Aerosols (fine particles in the air) are the other main human-made climate forcing. Their effect is more complex. Some "white" aerosols, such as sulfates arising from sulfur in fossil fuels, are highly effective and thus reduce solar heating of the earth: however, black carbon (soot), a product of incomplete combustion of fossil fuels, biofuels and outdoor

biomass burning, absorbs sunlight and thus heats the atmosphere. This aerosol direct climate forcing is uncertain by at least 50 percent, in part because aerosol amounts are not well measured and in part because of their complexity.

Aerosols also cause an indirect climate forcing by altering the properties of clouds. The resulting brighter, longer-lived clouds reduce the amount of sunlight absorbed by the earth, so the indirect effect of aerosols is a negative forcing that causes cooling.

Other human-made climate forcings include replacement of forests by cropland. Forests are dark even with snow on the ground, so there removal reduces solar heating.

Global Warming

Global average surface temperature has increased about 0.75 degree С during the period of extensive instrumental measurements, which begin in the late 1800s. Most of the warming, about 0.5 degree C, occurred after 1950. The causes of observed warming can be investigated best for the past 50 years, because most climate forcings were observed then, especially since satellite measurements of the sun. stratospheric aerosols and ozone began in the 1970s. Furthermore, 70 percent of the anthropogenic increase of greenhouse gases occurred after 1950.

The most important quantity is the planetary energy imbalance. This imbalance is a consequence of the long time that it takes the ocean to warm. We conclude that the earth is now out of balance by something between 0.5 and one watt per square meter - that much more solar radiation is being absorbed by the earth than is being emitted as heat to space. Even if atmospheric composition does not change further, the earth's surface will therefore eventually warm another 0.4 to 0.7 degree С

Most of the energy imbalance has been heat going into the ocean. Sydney Levitus of the national Oceanic and Atmospheric Administration has analyzed ocean temperature changes of the past 50 years, finding that the amount of heat required to melt enough ice to raise sea level one meter is about 12 watt-years (averaged over the planet), energy that could be accumulated in 12 years if the planet is out of balance by one watt per square meter.

The Time Bomb

The U.N. established an Intergovermental Panel on Climate (IPCC) with responsibility for analysis of global warming. The IPCC has defined climate-forcing scenarios, used these for simulations of 21^s-century

climate, and estimated the impact of temperature and precipitation changes on agriculture, natural ecosystems, wildlife and other matters. The IPPC estimates sea-level change as large as several tens of centimeters in 100 years, if global warming reaches several degrees Celsius.

These moderate climate effects, even with rapidly increasing greenhouse gases, leave the impression that we are not close to dangerous anthropogenic interference. I will argue, however, that we are much closer than is generally realized, and thus the emphasis should be on mitigating the changes rather than just adapting to them.

The dominant issue in global warming, in my opinion, is sea-level change and the question of how fast the ice sheets can disintegrate. A large portion of the world's people lives within a few meters of sea level. The need to preserve global coastlines sets a low ceiling on the level of global warming that would constitute dangerous anthropogenic interference. How fast will ice sheets respond to global warming? The IPPC calculates only a slight change in the ice sheets in 100 years; however, the IPPC calculations include only the gradual effects of change in snowfall, evaporation and melting. Yet I suspect that significant sea-level rise could begin much sooner if the planetary energy imbalance continues to increase. It seems clear that global arming beyond some limit will make a large sea-level change inevitable for future generations. And once large-scale ice-sheet breakup is under way. it will be impractical to stop.

1 argue that the level of dangerous anthropogenic influence is likely to be set by the global temperature and planetary radiation imbalance at which substantial deglaciation becomes practically impossible to avoid.

A Brighter Future

I have developed a feasible scenario with lower climate forcings. In addressing air pollution, we should emphasize the constituents that contribute most to global warming. Methane offers a great opportunity. If human sources of methane are reduced, it may even be possible to get the atmospheric methane amount to decline, thus providing a cooling that would partially offset the carbon dioxide increase.

Actions needed to reduce methane, such as methane capture at landfills and at waste management facilities and during the mining of fossil fuels, have economic benefits that partially offset the costs.

Improved energy efficiency and increased use of renewable energies might level carbon dioxide emissions in the near term. Long-term reduction of carbon dioxide emissions is a greater challenge, as energy use

will continue to rise. Progress is needed across the board: continued efficiency improvements, more renewable energy, and new technologies that produce little or no carbon dioxide or that capture and sequester it.

How can I be optimistic if climate is closer to the level of dangerous
anthropogenic interference than has been realized? If we compare the
situation today with that 10 to 15 years ago. We note that the main
elements required to halt climate change have come into being with
remarkable rapidity. I realize that it will not be easy to stabilize green­
house gas concentrations, but I am optimistic because I expect that
empirical evidence for climate change and its impacts will continue to
accumulate and that this will influence the public, public-interest groups,
industry and governments at various levels. The question is: will we act
soon enough? Scientific American. March 2004

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