BIOMES OF THE EARTH - OCEANS Phần 5 pptx

Among IPCC scientists the consensus is that human activ￾ities are releasing high levels of greenhouse gases into the

atmosphere, and this is causing an “enhanced greenhouse

effect” that is overheating the planet.

What is the greenhouse effect? Some of the gases that

occur naturally in the atmosphere—carbon dioxide and

methane, for example—absorb infrared radiation emitted

from Earth’s surface. They are called greenhouse gases

because, like the glass in a greenhouse, they absorb outgoing

infrared radiation and trap heat energy as they do so. On a

sunny day in winter, for example, the air in a greenhouse

becomes much warmer than the air outside, partly because of

this effect. On a global scale greenhouse gases trap heat in the

atmosphere and warm Earth’s surface.

The greenhouse effect is a natural process that has been

happening through much of the planet’s life. Without it, the

Earth today would probably be at least 54°F (30°C) cooler.

The problem lies in human activities “enhancing” the green￾house effect. When people burn large quantities of fossil

fuels—oil products, natural gas, coal, and so on—the activity

releases extra carbon dioxide into the atmosphere. This

increases the greenhouse effect, trapping more heat energy in

the atmosphere, and thus slightly warming the planet.

By analyzing the record of carbon dioxide trapped in polar

ice over the last few hundred years, scientists have discovered

that atmospheric carbon dioxide levels have risen by one￾quarter in the last 150 years. Recent temperature measure￾ments across the globe reveal that the 1990s were the hottest

decade since records began. Global warming appears to be

happening. Since the late 1990s, for example, the thickness

and coverage of Arctic sea ice has declined—perhaps an early

warning sign of global warming.

In 1998 many coral reefs across the Indian Ocean turned

white. This “coral bleaching” comes about when coral polyps

eject their partner algae (see “Coral grief,” pages 213–215).

The bleaching event can be enough to kill the coral polyps

that build the reef.

The 1998 coral bleaching event coincided with the

1997–98 El Niño, when surface water temperatures in parts of

the Indian and Pacific Oceans rose by 1.8 to 3.6°F (1 to 2°C)

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ATMOSPHERE AND THE OCEANS 93

above the seasonal normal, enough to cause some polyps to

eject their algae. Some scientists suspect that El Niño years

may become more frequent and more intense as global

warming worsens.

In their 2001 report the IPCC made their best estimate on

climate change, predicting that Earth’s surface would warm

by 5.2°F (2.9°C) during the 21st century. If this occurs, then

sea levels will probably rise by about 20 inches (50 cm) on

average. Most of this rise will come about through seawater

expanding slightly as it warms. Such a sea-level rise would be

sufficient to threaten low-lying countries. Much of

Bangladesh, for instance, is less than six feet (1.8 m) above

high tide levels, and many of the Maldives’ islands of the

Indian Ocean rise to only three to six feet (0.9–1.8 m) above

the current highest tides.

In any case, global warming by an enhanced greenhouse

effect is likely to make weather patterns more extreme and

unpredictable. Ocean currents, changing direction only

slightly, would bring heat and moisture to new locations and

deny it to others that currently receive it. Storms may

become more intense and droughts more severe.

The best approach to counter human-induced global

warming is to curb the release of greenhouse gases. But many

countries are acting too little and too late. The United States,

for example, has refused to sign up to the 1997 Kyoto Proto￾col to cut greenhouse gas emissions. By June 2005, represen￾tatives of more than 140 countries had signed this

international treaty. On average, each person in the United

States still produces, through the products and services they

consume, about twice as much greenhouse gas as each per￾son in Europe.