What is the Earth’s climate, what influences it, and how is climate different from weather?

Earth’s climate is a complex system influenced by interactions involving the oceans, the Sun, the atmosphere, aerosols, land use, human activity and other factors. Technically, climate is defined as the weather conditions resulting from the mean or average state of this complex atmosphere-ocean-land system which persist over months, seasons, decades or centuries. One way to think of weather is that it is what determines what clothes we wear outside on a given day. Climate is what determines what crops we plant for a given region and when we plant and harvest them.

Source: Modified from Climate Literacy: The Essential Principles of Climate -March 2009 U.S Climate Change Science Program

Oceans exert a major influence on climate by dominating Earth’s energy and water cycles. Covering 70% of Earth’s surface, oceans the capacity to absorb large amounts of solar energy. Heat and water vapor are redistributed globally through density-driven ocean currents and atmospheric circulation. Changes in ocean circulation caused by tectonic movements or large influxes of fresh water from melting polar ice can lead to significant and even abrupt changes in climate, both locally and on global scales.

The amount of solar energy absorbed or radiated by Earth is modulated by the atmosphere and depends on its composition. Greenhouse gases— such as water vapor, carbon dioxide, and methane— occur naturally in small amounts and absorb and release heat energy more efficiently than abundant atmospheric gases like nitrogen and oxygen. Small increases in carbon dioxide concentration have a large effect on the climate system.

The abundance of greenhouse gases in the atmosphere is controlled by bio-geochemical cycles that continually move these components between their ocean, land, life, and the atmosphere. The amount of carbon dioxide in the atmosphere is reduced through accumulation of marine sediments on the sea floor and the accumulation of plant biomass. The amount of carbon dioxide in the atmosphere is increased through deforestation, the burning of fossil fuels as well as through other processes.

Source: PhysicalGeography

Airborne particulates, called “aerosols,” also have a complex effect on Earth’s climate: they can cause both cooling, by reflecting incoming sunlight back out to space, and warming, by absorbing and releasing heat energy in the atmosphere. Small solid and liquid particles can be lofted into the atmosphere through a variety of natural and man-made processes, including volcanic eruptions, sea spray, forest fires, and emissions generated through human activities such as the combustion of coal and other fossil fuels.

The interconnectedness of Earth’s systems means that a significant change in any one component of the climate system can influence the equilibrium of the entire Earth system. Positive feedback loops can amplify these effects and trigger abrupt changes in the climate system. These complex interactions may result in climate change that is more rapid and on a larger scale than can be predicted by current climate models.

What is Climate Change?

Muir Glacier Bay, Alaska 1941

Muir Glacier in Glacier Bay National Monument 2004

Climate change is a significant and persistent change in an area’s average climate conditions or their extremes. Scientific observations indicate that Earth’s climate has changed in the past, is changing now, and will change in the future. The magnitude and direction of this change is not the same at all locations on Earth

Some factors contributing to climate change caused by natural forces, and others are caused by human activity such as the combustion of fossil fuels and changing land use patterns. Seasonal variations and multi-year cycles (for example, the El Niño Southern Oscillation) that produce warm, cool, wet, or dry periods across different regions are a natural part of climate variability and do not represent climate change.

Based on evidence from tree rings, other natural records, and scientific observations made around the world, Earth’s average temperature is now warmer than it has been for at least the past 1,300 years. Average temperatures have increased markedly in the past 50 years, especially in the North Polar Region.

Natural processes driving Earth’s long-term climate variability do not explain the rapid climate change observed in recent decades. The only explanation that is consistent with all available evidence is that human impacts are playing an increasing role in climate change, and that future changes in climate may be rapid compared to historical changes.

Natural processes that remove carbon dioxide from the atmosphere operate slowly when compared to the processes that are now adding it to the atmosphere. Thus, carbon dioxide introduced into the atmosphere today may remain there for a century or more. Other greenhouse gases, including some created by humans, may remain in the atmosphere for thousands of years.