CLIMATE AND wEATHER are not synonymous. Weather is the condition that prevails at a given moment and may change within days or even hours. At moments of meteorological fluidity, weather can change in minutes. Weather is variable, whereas climate is constant under ordinary circumstances. This is so because climate is the average of weather over long durations. Climate may change rapidly, as it did at the end of the Cretaceous era, but the circumstances that change climate rapidly are unusual. When climate changes rapidly Earth is in crisis. At the end of the Cretaceous, for example, a gigantic meteor impacted Earth, ejecting a huge cloud of debris and dust and touching off widespread forest fires. The debris, dust, and ash from this catastrophe blocked out sunlight, cooling Earth. These are not ordinary circumstances and most of the time, climate holds steady.

Climate arises from the interaction of several factors: latitude, proximity to oceans or mountains, altitude, radiation from the Sun, ocean currents, wind, continental drift, the greenhouse effect, volcanic activity, radioactivity of Earth's core, photosynthesis, transpiration, eccentricity of Earth's orbit around the sun, the tilt of Earth's axis, rainfall, and the reflection of sunlight from clouds, snow, and ice. Earth has many climates, ranging from desert to lush rainforest. Temperature and rainfall vary widely by locale. Near the equator, the range of temperatures is small and rainfall abundant. Away from the equator, temperatures and rainfall vary considerably. Humans have adapted to many climates. In contrast, many other animals are adapted to a single climate, and change threatens them.

The factors that shape climate begin with the Sun, for without it, Earth would be a dark, frozen, lifeless rock. The Sun's radiation, which Earth absorbs as heat, is 2.5 times greater at the equator than at the poles. Equatorial waters play a crucial role as a vast reservoir of heat. Near the equator, the ocean absorbs as much as 98 percent of the sun's rays, reflecting only 2 percent back into space. Warm equatorial waters flow toward the poles. The Gulf Stream, for example, carries equatorial water and warm air into the North Atlantic Ocean before cooling near the North Pole. In the past, equatorial currents have been warm enough to melt the ice caps, heating the climate.

Earth's tilt on its axis magnifies the effect of the sun's radiation. An Earth that did not tilt on its axis would not have seasons, because each latitude would receive a constant amount of sunlight and therefore a constant amount of heat year round. Earth is, however, tilted 23.5 degrees on its axis, causing it to receive varying amounts of sunlight and imparting seasons to the planet. When the Northern Hemisphere is tilted toward the Sun, this portion of Earth experiences summer, and the Southern Hemisphere, receiving less Sun that the north, experiences winter. When the Southern Hemisphere is tilted toward the Sun, the reverse is true.

the greenhouse effect

The greenhouse effect likewise amplifies the effect of the Sun's radiation. Greenhouse gases—carbon dioxide (CO2), methane, and water vapor are examples—trap sunlight in the atmosphere. Without any greenhouse gases, sunlight would pass through the atmosphere and strike Earth, which would absorb a portion of the sunlight. (Land absorbs less sunlight than water.) The rest would rebound from Earth as infrared radiation, passing out of the atmosphere and into space. Greenhouse gases do not, however, permit infrared radiation to pass into space, but rather absorb it as heat, in turn heating the atmosphere. Of the greenhouse gases, methane breaks down in the atmosphere after a few decades. CO2, however, may linger centuries in the atmosphere.

Earth produces CO2 through volcanic eruptions, spewing large quantities of it into the atmosphere. Since the Industrial Revolution, humans have increased the concentration of CO2 by burning fossil fuels. Humans are adding CO2 to the atmosphere faster than natural processes can reduce its concentration. As the concentration of CO2 in the atmosphere increases, so does temperature.

Counterbalancing the effect of volcanoes in increasing CO2 in the atmosphere, three factors reduce CO2 and so cool Earth. First, CO2 dissolves in rainwater to form carbonic acid, taking a portion of the gas out of the atmosphere. Second, the ocean absorbs CO2. Microorganisms in the ocean convert CO2 into carbonates, taking the gas out of circulation. Third, photosynthetic algae and plants consume CO2 during photosynthesis, converting it into sugars. Plants also affect the climate through transpiration, a process that adds water vapor to the atmosphere.

Water vapor traps more heat than either methane or CO2. Like methane, water vapor does not persist in the atmosphere, but rather falls to the ground as rain. The clouds that carry water vapor reflect sunlight back into space without letting it penetrate to Earth. Water vapor, therefore, has a complex effect on the climate. Water vapor is a greenhouse gas that warms Earth, but the clouds that carry water vapor cool the planet by blocking out sunlight. Clouds cover half the planet, reflecting 30 percent of sunlight, thereby cooling Earth.

Rain affects climate in several ways. It is abundant at the equator and at 30 degrees latitude, where it drenches Earth in monsoons. Rain is also plentiful at the windward side of mountains. As they rise to cross a mountain, clouds cool and release water vapor as rain. On the leeward side of mountains, clouds have little water vapor left to discharge as rain, and so the climate is arid. The climate is likewise arid between 15 and 30 degrees latitude, where air at low pressure prevents clouds from rising, cooling, and releasing their water vapor as rain. Rain sustains the growth of plants. Rapidly growing rainforests consume large amounts of CO2, though humans are chopping them down at an unsustainable rate.

Heat from Earth's core augments the heat supplied by the sun. Radioactive elements in the core decay over time, converting mass into energy, just

A cold climate reinforces itself with the accumulation of snow and ice, which reflect sunlight back into space.

as the sun and nuclear reactors do. This energy is in the form of heat and pressure. Pressure forces heat, in the form of molten rock, toward Earth's surface. Volcanic eruptions, in addition to spewing huge amounts of CO2 into the atmosphere, transfer molten rock from inside Earth to the surface, where molten rock liberates its heat.

Despite the fact that volcanoes liberate heat and release CO2, their effect on the climate is not always in the direction of higher temperatures. Volcanoes also spew debris, dust, and ash into the atmosphere. These particles block out the Sun and so may cool Earth. The eruption of Mount Tambora, in 1815, ejected huge clouds of debris into the atmosphere. Feeling the full effects of the eruption, 1816 was so cold that it is remembered as the year that had no summer.

Wind affects climate by carrying warm or cool air across the land. Warm air originates at the equator and follows ocean currents to higher latitudes. Cool air, originating at the poles, blows to lower latitudes. When warm and cool air meet, warm air rises and cool air sinks. As warm air rises, it cools, and sheds its water vapor as rain. The areas along contrasting weather fronts are, therefore, places of abundant rainfall.

In their restlessness, the continents affect climate. Earth is not a static entity as was once believed. Rather, the continents wander across Earth, changing their position and their orientation toward one another. When continents have moved toward the poles, their climates have cooled. Only when Antarctica, for example, moved to the South Pole did it acquire glaciers. In contrast, when continents gather near the equator, as Pangea did, the climate becomes balmy. Plants grew in abundance and herbivores became large, setting the conditions for the evolution of the dinosaurs.

Nor is Earth's orbit constant. In the 17th century, German astronomer Johannes Kepler demonstrated that Earth's orbit, as is true of the orbits of all the planets, is an ellipse. At one extreme, the ellipse is pronounced, and Earth is nearest the Sun at the closest approach of the ellipse and furthest from the sun at the greatest distance of the ellipse. The climate in these instances alternates between warmth when Earth is near the sun, and cold when Earth is far from the Sun. At the other extreme, Earth's orbit, though still an ellipse, is nearly circular. Earth, being roughly the same distance from the Sun at every point in its orbit, receives roughly the same amount of sunlight year round, and so has a uniformly warm climate.

A cold climate reinforces itself through the accumulation of snow and ice, both of which reflect sunlight back into space. By this mechanism, Earth, covered with snow and ice, cools because it reflects, rather than absorbs, heat in the form of sunlight. At the culmination of this feedback loop is an ice age, with Earth covered in glaciers. However, a warm climate can also reinforce itself through the accumulation of CO2 in the atmosphere. At the culmination of this feedback loop CO2 accumulates until all snow and ice have melted. The succession of climates, with varying temperatures, makes clear that climate is not static, but changes over time. The current climate is a warm interlude at the end of the Pleistocene ice age. Whether the climate will stay warm or descend into another ice age remains open to question.

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