Climate Change & Global Warming
Technological improvements such as satellites and moored ocean instruments now allow scientists to observe and monitor shifts in climate around the globe. These are supported by a network of more traditional, direct measurements of many factors, such as temperature, precipitation, wind speed and direction, which "ground-truth" satellite measurements. New networks such as the Global Ocean Observing System, or GOOS, will improve these measurements. While the causes of climate change are not yet fully understood, it is clear that they typically have a global-wide signal, and are the result of complex interactions between the oceans, atmosphere, and land. It also is apparent that these changes impact the structure of marine ecosystems and marine populations.
How Do We Know That Climate Has Changed in the Past? Much of climate change is comparatively small and happens over extremely long time scales. Unfortunately our base of direct observations of indicators of climate change - such as temperature and precipitation - are relatively short, about 100-200 years at most. So scientists use proxies to reconstruct past climate. These include records from tree rings, corals, glaciers, and sediments. Proxy time series are carefully compared to direct measurements from recent times to corroborate their utility as indicators of climate change.
Sophisticated computer models are used to understand the mechanisms of past climate change, and to predict future climate scenarios. These models are refined by comparing results with ocean or atmospheric measurements. Scientists have developed indices of climate change. These summarize a large amount of information, sometimes from a number of factors, into a single index that tracks climate variability. Climate indices help confirm that change is real and facilitate the monitoring of climate change. A popular example is the Southern Oscillation Index, or SOI, which compares measurements of atmospheric pressure at sea level at two locations to indicate El Niño events.
Does the Climate Vary Naturally?
Yes, much of the variability or change in climate is natural. There is ample evidence that the earth's climate has fluctuated for centuries, long before humans became a dominant force on the planet. On geological time scales, for example, everyone has heard of "ice ages". On shorter time scales, El Niño and La Niña events are examples of climate fluctuations that have occurred naturally for centuries. The source of this variability is probably a constant redistribution of energy, in the form of heat, moisture, and momentum, between the atmosphere, ocean, land, and ice. Another natural source of variability is volcanic activity, which at times adds large amounts of sulfuric gases and dust into the atmosphere, causing widespread cooling for a few years. Climate variability may also be driven by slight fluctuations in the influences of the sun on the Earth. These include variations in the energy output of the sun, in the Earth's orbit around the sun, in the Earth's axis orientation, and in its rate of rotation.
Analyses of proxy climate data, such as lake and ocean sediments, have shown that major shifts in regional climate patterns may occur over very short periods of time, even less than a decade. Significant ecological consequences are often associated with these rapid climate shifts. Entire terrestrial habitats and fauna or marine fish populations have been observed to shift over a few years in response to changes in measurable physical processes that occurred long before humans were involved.