Introduction to Climate Change
Understanding climate change begins with understanding the difference between weather and climate. Weather refers to atmospheric conditions that occur locally over a short period of time, such as rain, snow, clouds, or wind. Climate, however, refers to regional or global average patterns of temperature, humidity, and precipitation over longer time periods such as seasons, years, decades, or centuries. Climate is influenced by the movement of heat and moisture by air and ocean currents, which can affect the temperatures, precipitation, humidity, soil moisture, surface water levels, groundwater levels, and even storm events of a particular region.
Climate change refers to long-term changes in the average weather patterns that define the Earth's global, regional and local climates. Often when we think of climate change, we think of the physical effects of climate change such as heatwaves, sea level rise, and heavy rainfall, and the impacts on communities and the environment, such as floods and droughts.
Some climate change is influenced by naturally occurring changes in the Earth's temperature. For at least the last million years, the Earth has experienced natural cycles of cooling and warming, taking the Earth into ice ages and warming it up again. Each of these naturally-occurring cycles has taken approximately 100,000 years to complete.
However, today's climate is changing much more rapidly because of greenhouse gas emissions released from burning fossil fuels, deforestation, wetland loss, and other human activities that are causing the Earth's average temperature to heat up much faster than it would naturally.
The Greenhouse Effect, Greenhouse Gases, and Global Warming
Energy from the sun, or solar radiation, travels through space and enters the Earth's atmosphere, which forms a kind of warm blanket around the Earth. Some solar radiation remains trapped in the atmosphere under this blanket and is absorbed by land, air, and surface waters, while some solar radiation is reflected by land, ice, or clouds and escapes back out into space. When the Earth reflects the same amount of energy as it absorbs, the balance keeps the planet's average temperature fairly stable. This is called the greenhouse effect, and is essential for sustaining life on Earth. The revolution of the Earth around the sun and the tilt of the Earth on its axis create variation in the intensity of solar radiation received by different sections of the planet and drive naturally occurring cycles of planetary cooling and warming. This variation over time is what sends the Earth into ice ages and then into shorter warming periods between these ice ages. However, these natural climate cycles can be disrupted by conditions that cause an excessive amount of solar radiation to get trapped in the Earth's atmosphere and throw off the natural balance.
Greenhouse gases (GHGs) are heat-trapping gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N20), that are naturally present in the Earth's atmosphere in small amounts and help form the atmospheric blanket at the ideal thickness. CO2 is vital for plants to carry out photosynthesis and produce oxygen for humans and animals to breathe. When excess GHGs are generated by human activities and accumulate in the atmosphere, they increase the thickness of the atmospheric blanket, trapping solar radiation in the Earth's atmosphere. GHGs absorb and transmit that radiation back to Earth as heat. The more GHGs accumulate in the atmosphere, the thicker the blanket becomes, trapping more heat inside and creating global warming.
Global warming causes the Earth's land and oceans to absorb heat energy and warm more quickly than would happen naturally. Global warming is made worse by the melting of polar ice and glaciers. Because of the albedo effect, light-colored snow and ice act as mirrors to reflect solar radiation back out into space. As ice sheets and glaciers melt with increased global warming, reducing the amount of ice and snow covering, less of the solar radiation is reflected and more is absorbed into the darker ocean waters, creating a vicious cycle that warms the planet even more and melts even more polar ice and glaciers.
Global warming has been rapidly accelerating in recent decades due to excess emissions of GHGs from human activities, especially burning of fossil fuels and deforestation. While the term "global warming" refers to the long-term warming of the planet from excess GHG emissions, the term "climate change" refers to the broader range of changes that are happening to the earth, including sea level rise, shrinking glaciers, ocean acidification, desertification, more intense storm activities, and shifts in plant blooming times. Increased warming of the Earth's oceans and the atmosphere is driving climate change by affecting the way heat and moisture are moved through the climate system by air and ocean currents, changing patterns of precipitation, temperatures, storms, and even sea level all around the globe.
Oceans cover 70% of the Earth's surface and can absorb large amounts of solar energy. Heat and moisture are moved by ocean currents and atmospheric circulation, influencing the climate around the planet.
The oceans have a large capacity for absorbing and holding heat. For decades, the oceans have taken up more than 90% of excess heat from the atmosphere. If not for the ability of Earth's oceans to absorb such large amounts of heat, the atmosphere would warm at a much faster rate. The Intergovernmental Panel on Climate Change 6th Assessment Report (AR6) finds that the rate of ocean warming observed since 1971 will at least double by the year 2100, even if GHG emissions are reduced. Ocean temperature plays an important role in the Earth's climate system, particularly sea surface temperature (SST). SST has risen as oceans absorb more heat from the atmosphere, resulting in changes to ocean currents and circulation, as well as sea level rise. SST has a significant impact on climate. As SST rises, increased evaporation leads to a greater concentration of water vapor over the oceans, influencing weather systems that produce precipitation or contribute to drought conditions. Heat from water at the ocean's surface provides tropical storms and hurricanes with energy and influences weather patterns.
The U.S. National Oceanographic and Atmospheric Administration (NOAA) reports that over the last two decades the Atlantic Ocean along the northeastern United States (U.S.) has warmed faster than 99% of the global ocean. Coastal temperatures along the northeastern U.S. increased by 0.06°F per year from 1982 to 2016. Ocean temperatures over the Northeastern continental shelf are expected to warm by up to 0.76°F per decade by 2070.
Ocean circulation plays an important role in climate through the movement of heat, freshwater, nutrients, and carbon. The Atlantic Meridional Overturning Circulation (AMOC) moves warm water from the tropics poleward and cold water from the poles towards the equator, largely influencing the climates of North America and Europe. However, warming ocean temperatures and changes in ocean salinity due to an influx of freshwater from melting sea ice and glaciers, can slow down the AMOC. The amount and velocity of heat transported by AMOC can be a cause for extreme weather and changes to precipitation patterns experienced by these continents.
Increasing ocean temperatures can also create disturbances to marine ecosystems and the loss of essential habitats for marine fish and mammals. The loss of coral reef habitat from coral bleaching is a result of warming ocean temperatures. Pathogens, such as harmful algal blooms and bacteria, can spread more easily in warmer waters, putting global water supplies, human and animal health, and economies that depend on the fishing industry, at risk.
High levels of GHGs in the atmosphere are not only increasing global warming and intensifying climate change but are also threatening the health of ocean life and ecosystems. The oceans play a crucial role in absorbing heat, but also in regulating the amount of carbon dioxide in the atmosphere. The oceans have taken up almost 30% of total atmospheric CO2 emissions since 1980. CO2 reacts with ocean water to produce carbonic acid, which increases the acidity (lowers the pH) of ocean water. The more atmospheric CO2 levels rise, the more intensely ocean acidification will occur.
While the ocean's ability to absorb CO2 helps to reduce CO2 levels in the atmosphere, ocean acidification is impacting marine life and ecosystems, especially corals, plankton, shellfish, and other marine organisms with shells and skeletons composed of a mineral called calcium carbonate. Acidic water dissolves calcium carbonate, preventing these organisms from thriving and contributing to larger-scale impacts on ecosystems that rely on these organisms for survival.
Human Influence on Climate Change
Where do excess GHGs in the atmosphere come from? Over the last century, human activity has been and still is the largest source of GHG emissions in the atmosphere.
GHGs are released into the atmosphere largely through burning fossil fuels. CO2 is the most abundant GHG present in the atmosphere because fossil fuels, such as coal, oil, and natural gas are composed mainly of carbon. When these fuels are burned to produce electricity, for heating, or in vehicles with internal combustion engines, carbon is released into the atmosphere. That carbon combines with oxygen in the atmosphere to form CO2. GHGs such as CO2, CH4, and N20 are also emitted as a result of other human activities such as farming, mining, manufacturing, certain industrial processes, and even the decomposition of organic waste in landfills.
Forests, wetlands, peatlands, and similar ecosystems are known as carbon "sinks" because they naturally absorb and store carbon from the atmosphere in a process called carbon sequestration and storage. However, deforestation and the destruction of these ecosystems by human activity are causing the carbon stored in these natural sinks to be released, increasing atmospheric CO2 concentrations even more.