Look outside your window right now.  Is it raining or snowing?  Is the temperature hot or cold?  Does the air feel humid or dry?  These are all important elements of not just the weather, but of climate. Weather and climate are related, but distinct, concepts. While weather is the state of the atmosphere at any given moment—the temperature, the amount of rain, etc.— climate is the average weather condition over longer periods of time.

Climate affects all life on Earth. At the most basic level, climate impacts what species can live where, and what sort of measures people need to take in order to thrive in a given location.  Plants and animals depend on cues from climate to trigger activities like blossoming, egg-laying, and migration.  Meanwhile, knowledge of a regional climate tells people when to plant crops, if they should build houses that will withstand blizzards, and whether they’ll need to bundle up in the winter or if bikini season lasts all year.  When the climate changes, it impacts all life on Earth, as well as other non-biological Earth systems.  Here we will explore the processes that affect climate and the reasons that our climate has been warming since the mid-20^th century.

The Earth’s climate is a complex system controlled by several factors including solar activity, Earth’s reflectivity and orbit, and atmospheric greenhouse gas concentrations. The amount of radiation entering Earth’s atmosphere varies depending on natural variations in the sun’s energy output and cycles in the earth’s orbit, called Milankovitch cycles.  Milankovitch cycles are responsible for glacial and interglacial periods (i.e., ice ages) over long timescales (e.g., 100,000 years).  They are caused by variations in the shape of Earth’s orbit around the sun, the tilt of Earth’s axis, and a small wobble in that tilt, all of which influence the distribution of solar energy on Earth.

When solar radiation enters our atmosphere, the earth absorbs some of this energy, warming the planet, while some is reflected out towards space, cooling the planet. How much is absorbed versus reflected is determined by the planet’s albedo, or reflectivity.  A lighter colored surface such as ice or snow reflects more energy out towards space than does a darker surface such as a deep blue ocean or a dark green forest.  This can lead to a feedback loop, where changes in Earth’s climate cause greater or lesser snow coverage, leading to changes in Earth’s albedo, which in turn affects the climate.

Once reflected, some of the outgoing radiation is absorbed by atmospheric greenhouse gas (GHG) molecules, such as carbon dioxide and methane, in a process known as the greenhouse effect.  The GHG molecules then emit that absorbed energy in all directions (including back towards Earth and towards other GHG molecules which will absorb the energy again).  This process keeps the energy in our atmosphere for longer, warming it.  The warming of the atmosphere leads to warmer land surfaces and oceans, which impact life all around the globe.

Additional greenhouse gases released through human activities, such as the burning of fossil fuels, enhance the greenhouse effect, causing climate change—aka, global warming.  In recent years, temperatures have been increasing as a result of global warming, with global average temperatures of both land and ocean surfaces warming as much as 1°C since the 1850’s (that’s almost 2°F).  Researchers predict that it could increase by as much as 4°C (over 7°F) by 2100. This may not seem like a lot, but keep in mind that this is global climate, not just the day’s weather. Small changes in annual average global temperatures can have huge impacts on everything from ice sheet formation to the timing of when birds lay their eggs.

Changes in temperature can also influence rain and snowfall patterns (i.e., precipitation) in surprising ways.  Warmer temperatures mean faster evaporation from oceans and bodies of water, leading to more water in the atmosphere.  Furthermore, a warmer atmosphere can hold more moisture than a colder atmosphere. This higher moisture content in the atmosphere can lead to more precipitation events (e.g., snowstorms, monsoons) of a greater intensity.  At the same time, regions that are already struggling with water availability may not reap the benefits of this moist atmosphere.  Climate models predict that some dry regions will see higher rates of evaporation but less rainfall, leading to regional droughts.  The temperature can also influence how precipitation falls, as either rain or snow, which can lead to flooding and cause water shortages for regions that rely on snowmelt for their water source.

A changing climate even affects the physical landscape (e.g., when Earth’s average temperature goes up, glaciers melt and sea level rises correspondingly) and of course the biological world (e.g., local temperature shifts can change habitats, alter the growing season, and ultimately lead to species movement, migration, or even extinction).  Though climate has changed over the course of Earth’s history — for example, a balmy climate 55 million years ago led to an ice-free North Pole — multiple lines of evidence demonstrate that recent changes are due to human activity.  Follow the stories below to see how climate has impacted life on Earth and how it is projected to change in the near future.