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Why 350?: An Introduction to the Greenhouse Effect

By Austen Needleman

When we hear about climate change, we also hear plenty of numbers: the Paris Agreement’s goal to limit warming to 1.5° Celsius, projections that sea levels will rise 10-12 inches in the next 30 years, and many more. There is also, of course, our organization’s name: 350. 

Where does the name 350 come from, and how does it connect to climate change?

350 is the safe upper limit of carbon dioxide (CO2) in our atmosphere, as measured in parts per million (ppm). We’re currently at 420ppm and rising about 2 ppm per year. While the amount of CO2 in the atmosphere has cycled throughout the history of the Earth, it has never exceeded 300 ppm before the 20th Century. These unprecedented levels of CO2 have led to similarly unprecedented changes in the Earth’s climate. 


Atmospheric carbon dioxide leads to climate disruption via the greenhouse effect. Alongside other atmospheric gases, including methane, nitrous oxide, and fluorinated gases, CO2 forms a heat-trapping blanket around the Earth. Like a good winter coat, this effect is partially responsible for keeping the Earth at a habitable temperature. However, as we add more CO2 and other greenhouse gases to our atmosphere by burning fossil fuels, excess heat cannot escape into space, warming the planet.

This thickening blanket has already increased global temperatures by about 1.1° Celsius (~2° Fahrenheit) since 1880. Given the complexity of Earth’s climate systems, this change in temperature has led to a variety of impacts, including larger and more destructive fires, floods, droughts, and more intense storms.

For the first time in the history of the planet, atmospheric CO2 recently exceeded 420 ppm, and this number continues to to increase by 2-3 ppm every year. Burning fossil fuels, including coal, petroleum products, and methane gas (also known as “natural gas”), is the main factor responsible for this change. While renewables are gaining traction, fossil fuels remain a key source of energy for powering our appliances, cooking our food, and getting from place to place. In addition to their contribution to warming via the greenhouse effect, extracting and burning fossil fuels also causes tremendous damage to ecosystems across the planet and produces toxic pollution.

The number 350 is a guide and a way toward a healthier planet. There is still time to avoid the worst impacts of climate change by reducing atmospheric CO2! Doing so requires a global mass movement to stop extraction and use of fossil fuels. SanDiego350 is part of that movement – check out our Take Action page to see how you can become a part of it too!