What is climate change, and why does it happen?

Climate and Weather

Before we can understand what climate change is, we have to understand what climate is! A good place to start is by thinking about the difference between climate and weather.

Weather is the day-to-day or hour-to-hour change in variables like temperature, rainfall, and wind.  It sometimes feels like the weather changes from minute to minute! 

Climate, on the other hand, is the average of weather variables over the long term: technically, at least three decades. In Ithaca, New York, we live in a climate with four seasons, with warm summers and cold, snowy winters, and we average about 40 inches of rain per year.

What’s the weather like right now on the west side of Cayuga Lake in Ithaca, New York?

The data you see here come from a weather station at the Museum of the Earth, a few miles down the road from the Cayuga Nature Center. They show the influence of Cayuga Lake on the microclimate here.  A microclimate is the climate of a very small area—it can be snowing up on Ithaca’s East Hill while it’s raining in downtown Ithaca. 

Even different hillsides can have different microclimates.  Wineries and other agricultural businesses can use information from weather stations to their advantage, for example, to decide the best location to plant grapes to protect them from frost damage.

While the effect of global climate change on a particular location will depend on local factors that influence its microclimate, we can average together data from the many weather stations around us to see how climate change is playing out.

Earth’s climate has changed in the past

Earth’s climate has changed many times over its 4.5 billion year history, at times gradually and at other times rapidly.  Natural causes include changes in the Earth’s orbit around the sun, changes in the output of energy from the sun, and volcanic activity.

Watch these videos to learn more:

Climate affects life and life affects climate

Life on Earth has affected climate just as climate has affected life. For example, around 300 million years ago, expanding forests absorbed heat-trapping carbon dioxide through photosynthesis. Many of these plants lived in swamps with surface water that was low in oxygen, so the plants didn’t decompose easily when they died. Instead, the plants were buried and the carbon they stored was not returned to the atmosphere—this eventually led to a cooling of the Earth. The plants fossilized after millions of years and turned into coal, effectively storing energy from the sun underground.

The diagram below shows the connections between life on Earth and carbon in the atmosphere, through the carbon cycle.

Carbon fluxes (flows) are expressed in gigatons (billion tons) of carbon per year. Yellow numbers are natural carbon fluxes, and red are human contributions. White numbers indicate stored carbon, in gigatons.

Today, human activity is warming the planet very rapidly

Since the dawn of the Industrial Revolution about 200 years ago, humans have continued to increase the rate at which we burn fossil fuels such as coal, oil, and natural gas, releasing carbon dioxide (CO₂) into the atmosphere.  About half of the CO₂ added to the atmosphere since the beginning of the Industrial Revolution has been added since around 1980. We are warming our planet more rapidly than at any other time in human history.

What will New York State’s temperature be in the future?

To answer this question, scientists use computers to model the climate system. These models solve the equations of physics that govern the Earth’s climate, and they have inputs such as the amount of incoming solar energy and the presence of forests, water, ice, and clouds. They also include the results of human actions, such as greenhouse gas emissions from burning fossil fuels and clearing forests for farmland.

Of course, the level of greenhouse gases in the atmosphere in the future depends on the choices we make today. If we choose to reduce greenhouse gas emissions, future warming will be less severe than if we keep on emitting lots of greenhouse gases. Climate models use different emissions scenarios to reflect these choices. The graph below shows projections of future annual average temperatures, averaged over 32 different models, for New York State. The emissions scenarios are:

MEDIUM EMISSIONS: greenhouse gas emissions increase until the year 2040, after which they decrease.

HIGH EMISSIONS: greenhouse gas emissions continue to increase throughout the 21st century.