Climate Change Causes

The Science Behind Our Changing Climate

To gain an understanding of what is causing our climate to change, let us take a brief look at measurements that scientists such as those at NASA and the U.S. National Oceanographic and Atmospheric Administration (NOAA) have been tracking for many years.

Scientists are in agreement that the global climate is rapidly warming and that the causes are rooted in human activity, predominantly combustion of fossil fuels and land use changes such as deforestation. Some of the clearest evidence that global climate change is occurring and is being caused by human activity comes from carbon dioxide (CO₂) levels in the atmosphere. The following graph shows atmospheric CO₂ levels over the past 800,000 years:

Historical levels of carbon dioxide. This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements, provides evidence that atmospheric CO₂ has increased since the Industrial Revolution. (https://climate.nasa.gov/evidence/)

It is evident that while atmospheric CO₂ levels do naturally fluctuate, today’s levels are the highest in millennia. Carbon dioxide levels are much higher than they would otherwise be if humans were not burning so much fossil fuels (oil, gas, and coal), which give off CO₂ when they burn.

By studying hundreds of thousands of years of geological data, scientists have shown that temperatures increase when CO₂ levels increase, so the current spike in carbon dioxide is certain to result in a rapid additional increase in global temperature.

Historical temperature levels. Temperature histories from paleoclimate data (green line) compared to the history based on modern instruments (blue line) suggest that global temperature is warmer now than it has been in the past 1,000 years, and possibly longer. (Graph adapted from https://earthobservatory.nasa.gov/features/GlobalWarming/page3.php)

The paleoclimate record reveals that the current climatic warming is occurring much more rapidly than past warming events.

What is the Greenhouse Effect?

The greenhouse effect refers to the retention of the sun's warmth in the Earth's lower atmosphere by greenhouse gases. These gases (primarily carbon dioxide, methane, and nitrous oxide) act as a thermal blanket for the planet, warming the surface to what has been a life-supporting global average of 59 degrees Fahrenheit (15 degrees Celsius). The recent rapid increase in greenhouse gases has resulted in the atmosphere trapping more solar radiation and, in turn, raising the global temperature. There are three main sources of greenhouse gases:

• Carbon in Fossil Fuels: A major source of carbon in our atmosphere has been from the accumulation in the earth, over eons, of decayed organic matter from plants. Its longest-lasting deposits are coal, oil, and natural gas. Under natural conditions, these types of carbon slowly accumulated and remained stored in the Earth. However, over the past two centuries or so, humans have been extracting them and utilizing these “fossil fuels” for energy, by burning them for transportation, heating, and industrial purposes. Thus, carbon that took millions of years to be taken up from the atmosphere and accumulate underground gets released back into the atmosphere in a short period of time. The rate of extraction and burning has increased over recent decades, adding to atmospheric CO₂ levels.
• Plants and the Carbon Cycle: Changes in land use, such as deforestation, have reduced the overall number of green plants on earth available to take up carbon dioxide from the atmosphere. Green plants absorb CO₂ through their leaves and convert it to energy-rich organic compounds in the process of photosynthesis. Trees, for example, use those compounds to form their woody stems and roots. Thus, as plants grow, carbon gets stored in them. When they die, that carbon gets decomposed by microorganisms and fungi, with the result that a significant fraction gets stored in the soil in the form of organic matter. A portion of that carbon gets released back into the atmosphere as CO₂, where it can then be taken up again by plants. This circular process is known as the carbon cycle. But the loss of forests worldwide (deforestation) to other uses such as mining, agriculture, urban and suburban sprawl, has led to fewer forested acres to sequester and store the ever-increasing amount of CO₂ in the atmosphere, with less stored in plant tissue or in the soil.
• Methane Gas: Methane is a very powerful greenhouse gas that is released to the atmosphere as a result of the natural fermentation of organic matter (for example from swamps). However, increasingly it is being emitted by ruminants (e.g., cattle, sheep, goats, and their relatives) as the world's population expands its need for food, from  landfills, and from the extraction and leakage of natural gas from wells and pipelines. It has also been predicted and hypothesized that as temperatures rise, methane trapped by permafrost in northern boreal forests and methane trapped beneath the oceans will begin to be released into the atmosphere.

The vast majority of scientists agree that combustion of fossil fuels is the single largest driver of rising temperatures and climate change. If the release of CO₂ is not slowed, the costs to society of addressing its impacts will become staggering. The increasing availability of renewable forms of energy, such as solar, wind, and hydroelectric power, offers options for meeting energy needs. Fortunately, the cost of renewable energy can be competitive with gas and oil.

Banner graphic: https://www.nasa.gov/press-release/nasa-releases-detailed-global-climate-change-projections.