CLIMATE CHANGE GLOBAL WARMING the phenomenon of increasing

CLIMATE CHANGE

GLOBAL WARMING the phenomenon of increasing average air temperatures near the surface of Earth over the past one to two centuries. Climate scientists have since the mid-20 th century gathered detailed observations of various weather phenomena (such as temperatures, precipitation, and storms) and of related influences on climate (such as ocean currents and the atmosphere’s chemical composition). These data indicate that Earth’s climate has changed over almost every conceivable timescale since the beginning of geologic time and that the influence of human activities since at least the beginning of the Industrial Revolution has been deeply woven into the very fabric of climate change.

HUMAN ACTIVITIES ARE CHANGING THE CLIMATE • Rigorous analysis of all data and lines of evidence shows that most of the observed global warming over the past 50 years or so cannot be explained by natural causes and instead requires a significant role for the influence of human activities. Fingerprinting is a powerful way of studying the causes of climate change. This becomes obvious when scientists probe beyond changes in the average temperature of the planet and look more closely at geographical and temporal patterns of climate change. Different influences on climate lead to different patterns seen in climate records.

For example, an increase in the Sun’s energy output will lead to a very different pattern of temperature change (across Earth’s surface and vertically in the atmosphere) compared to that induced by an increase in CO 2 concentration. Observed atmospheric temperature changes show a fingerprint much closer to that of a long-term CO increase than to that of a fluctuating Sun alone. Scientists routinely test whether purely natural changes in the Sun, volcanic activity, or internal climate variability could plausibly explain the patterns of change they have observed in many different aspects of the climate system. These analyses have shown that the observed climate changes of the past several decades cannot be explained just by natural factors.

CAUSES AND EFFECTS OF CLIMATE CHANGE Glaciers are melting Sea levels are rising Wildlife is scrambling Cloud forests are dying

It has become clear that humans have caused most of the past century's warming by releasing heattrapping gases as we power our modern lives Their levels are higher now than at any time in the last 800, 000 years.

AREN'T TEMPERATURE CHANGES NATURAL? Volcanic eruptions and variations in solar radiation from sunspots. Human activity isn't the only factor that affects earth's climate. solar wind, and the earth's position relative to the sun also play a role. So do large-scale weather patterns such as El Niño. For thousands of years now, emissions of greenhouse gases to the atmosphere have been balanced out by greenhouse gases that are naturally absorbed. As a result, greenhouse gas concentrations and temperatures have been fairly stable, which has allowed human civilization to flourish within a consistent climate.

WHY DO GLACIERS MATTER? • Water has unique qualities that come into play in understanding the roles glaciers play in the earth system. • For example, water expands rapidly when it freezes an increase of about 9% by volume. Also, water enters the atmosphere through the familiar cycle of melting and evaporation, but water can also enter the atmosphere directly from a solid state through a process known as sublimation. Also, water the only substance that is lighter in its solid (ice) than its liquid state has more density as salt water than as fresh water.

CONSEQUENCES • The melting fresh water from glaciers alters the ocean, not only by directly contributing to the global sea level rise, but also because it pushes down the heavier salt water, thereby changing what scientists call the THC, or thermo (heat) haline (salt) circulation, meaning currents in the ocean. Water not only expands when frozen, but also when heated, and it is estimated that in the 21 st century the melting of ice caps and glaciers combined with thermal expansion of ocean water will cause an average increase of sea level of roughly a meter (a bit more than one yard).

• As the planet’s air conditioner, the polar ice caps impact weather and climate dynamics, such as the jet stream. Glaciers are also early indicators of climate changes that will have a somewhat more delayed impact on other parts Glaciers are sentinels of climate change. of the earth system. They are the most visible evidence of global warming today. In addition to raising sea water levels, widespread loss of glaciers will likely alter climate patterns in other, complex ways. For example, glaciers’ white surfaces reflect the sun’s rays, helping to keep our current climate mild. When glaciers melt, darker exposed surfaces absorb and release heat, raising temperatures.