How climate change works

In my last post, I addressed some common questions that farmers ask about climate change. Although I considered why the scientific information documenting climate change is trustworthy, I didn’t actually explain how climate change works. A savvy reader picked up on this and was dissatisfied that I didn’t present the relationship between increasing CO2 and global warming. In this post, I’ll correct that omission.

The CO2-temperature connection occurs through the “Greenhouse Effect”, a process that almost everyone has heard about but surprisingly few people can explain.

More CO2 in the atmosphere shows heat loss
Source: modified from The Climate Reality Project

Here’s how the Greenhouse Effect works. The sun warms the Earth, and the heat is then radiated back to space in the form of infrared waves. Ever notice the shimmering heat rising from a hot parking lot in the summer? That’s infrared radiation heading back to space. However, whenever an infrared wave bumps into a molecule of one of the “greenhouse gases” (CO2, methane, nitrous oxide or water vapor), its path to space is interrupted because it bounces off the gas molecule and heads off in some random direction. This means that 50% of the time, the infrared waves will go back toward Earth rather than continuing out to space. The higher the concentration of greenhouse gas molecules, the more molecules a wave will hit and the longer its heat will stay in the atmosphere while it is bouncing around. Simple!

The Greenhouse Effect has been around since Earth’s atmosphere was formed billions of years ago. By slowing the loss of heat after sunset, it makes life on Earth possible. Solar radiation and heat loss have remained roughly in balance over Earth’s long history as various other factors have caused CO2 and air temperature to cycle slowly up and down.

Temp and CO2 Concentration vs Age

Using data from ice cores, scientists have tracked these cycles over the past 800,000 years. These studies reveal that over prehistoric time, the concentration of CO2 in the atmosphere never exceeded 300 parts per million (ppm, dashed line in graph to the right). Now we are over 400 ppm. The rapid rise to this high value is not part of the previous natural cycle—it is just too far out of the usual range.

The orderly fluctuations of CO2 and temperature through prehistory ended in the late 1800’s when humans discovered the great things we can do by excavating and burning fossil fuels. Coal, oil, and gas are essentially concentrated carbon from ancient plants or microscopic organisms. When they are burned to power human activities, the carbon that was stored underground for eons is released into the atmosphere as CO2.

CO2 over time
Source: National Climate Assesment 2014

Industrialization, transportation and, changes in land use are behind the sharp rise in the concentration of CO2, methane (mostly from domesticated animals) and nitrous oxide (mostly from synthetic fertilizer). This can be easily seen in a graph with a timescale that covers only the past 2000 years. Because each infrared wave now runs into many more greenhouse gas molecules on its way out to space, it takes longer for the heat to escape, so the atmosphere warms.

Here’s the bottom line: Roughly the same amount of heat has been coming in from the sun, but the increased concentration of CO2, methane, nitrous oxide and water vapor slows the loss of this heat, causing the air to warm up. Increasing concentrations of greenhouse gases means slower heat loss and more warming.

We can see below that temperature and CO2 have increased virtually in lockstep. The smoking gun indicating human causation is that carbon emissions from fossil fuels have increased on the same timetable. The Greenhouse Effect explains why— more CO2, higher temperature.

Temp anomaly and carbon emissions
It’s hard to argue with the Greenhouse Effect. To deny that increased CO2 from burning fossil fuels is the major cause of global warming requires proposing another equally effective mechanism.

Source: IPCC AR5.

Some people have suggested that changes in solar radiation or volcanic activity are responsible, not human-caused increases in greenhouse gases. However, when changes in temperature due to these natural factors are graphed, neither solar or volcanic effects are correlated with the temperature increase seen since the late 1800s.

Humans have caused climate change, primarily by burning fossil fuels. Other than the Greenhouse Effect, no adequate mechanism for atmospheric warming has been proposed. Though it may be hard to accept that human activity is responsible for the climate crisis, knowing the cause immediately leads us to the solution: decrease the emission of CO2 and the other greenhouse gases.

We can limit the future impacts of climate change by embracing energy efficiency to reduce our demand for power, changing from internal combustion engines to electric ones, and switching to clean renewable energy. With tremendous recent progress in both commercial and residential energy efficiency and dramatic declines in the cost of wind and solar, this is more possible than ever before. Moreover, economists now say that it is far cheaper to make these changes than it is to clean up after the climate-related disasters that are increasing each year.

You can be part of the solution by being aware of the ways that climate change is impacting your garden and community, explaining how it works to others, and implementing or supporting climate-friendly practices and policies.

Learn more about climate change and gardening on the HGIC website.

Dr. Sara Via
Professor & Climate Extension Specialist,
UMD, College Park,

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