2 degrees Centigrade explained: what warming temperatures mean
About 12,000 years ago, at the start of the current geological epoch called the Holocene, the average global temperature was about 6 degrees centigrade lower than the average temperature then and now (i.e. 11 degrees Fahrenheit colder).
As a result, global mean sea level was 100 feet lower due to the increased amount of water captured in ice and snow.
Observations collected over the past 200 years indicate that since the 1800s, the global average temperature has increased by about 1.1 degrees centigrade. The global average temperature is a measurement of the temperature of the Earth’s surface, that is, of the first meters of the atmosphere and the oceans.
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The global average temperature is based on measurements from some 10,000 ground stations and numerous measurements at sea by ships and robotic floats. Although there has been variability from year to year, the global temperature has been steadily warming. The past 10 years have been the warmest years on record since measurements began in the 1800s.
The idea of limiting global average warming to 2 degrees Celsius measured above the 1850-1900 average was first discussed in the early 1990s to create a goal that the nations of the world could work together to achieve. .
This limit was first endorsed by the EU and then adopted by the global community in the 2009 Copenhagen climate accord. It was further reinforced at the 2015 Paris Climate Change Conference, where 195 countries agreed to report and reduce emissions that cause global warming.
Greenhouse gases and temperature
To support the 2 degree limit, climatologists developed models that linked greenhouse gas concentrations to global temperature. About half of emissions from human activities are absorbed by the oceans and terrestrial biosphere.
The remaining atmospheric concentrations of greenhouse gases prevent the evacuation of heat energy from the Earth’s surface to space.
Relatively simple models that balance the energy received from the sun with the amount escaping into space using greenhouse gas concentrations predict warming near the surface. It is currently estimated that doubling pre-industrial greenhouse gas concentrations by 280 ppm (parts per million) would raise the global temperature by 2.7 to 3.4 degrees Celsius. The current concentration is approximately 414 ppm.
Although it is difficult to be certain of the amount of future greenhouse gas emissions, one consensus estimate suggests that we could reach the 2 degree limit somewhere between 2050 and 2060. The global strategy is therefore to reduce greenhouse gas emissions to reduce atmospheric concentrations and thereby slow the rise in global temperature.
Temperature and kinetic energy
You may remember from high school science that temperature is a measure of molecular motion or kinetic energy. The flow of energy from the equator to the poles is how weather events are powered. The increase in the global average temperature measures an increase in the energy of the fluids that circulate on the Earth’s surface.
A limit like 2 degrees Celsius is small compared to the temperature fluctuations we observe over the course of a year. If you were to record daily temperature over a year, you would assemble a temperature distribution. You might experience a day above 95 degrees F every year. A warming of 2 degrees C would shift your distribution to higher temperatures by 3.6 degrees F.
After the change, some of the daily temperatures between 90 and 95 degrees F are moving to between 95 and 100 degrees F. So these higher temperatures, once considered extreme and infrequent, are becoming more common. The threat of climate change is not so much that the world gets a little warmer, but rather that the likelihood of extreme temperatures becomes more likely.
Uncommon extreme temperatures
Extreme temperatures are accelerating the decline in outdoor worker productivity and making some parts of the Earth hard places to live. Summer temperatures in England reaching record highs above 104 degrees F are one of many examples of unusual temperatures in 2022.
Sea level rise is caused by both spreading warmer waters and melting ice and snow, leading to tidal flooding and increased storm surges during coastal storms. Flooding in Miami during tidal events has become common, necessitating major infrastructure changes in the city.
Higher air temperatures capturing more moisture lead to increased precipitation which causes flooding. Some 10-15% of Pakistan was recently under water during 3 months of record rains.
Hurricanes can and have intensified rapidly due to a warmer ocean surface. Intensification leads to increased storm wind speeds and increased moisture content. Hurricane Harvey in Houston, TX and Florence, NC brought about 20% more moisture to shore due to warm surface waters. Heavy rainfall is the main cause of damage caused by tropical storms.
2 degree target
Nobel Prize-winning economist William Nordhaus has launched a cost-benefit analysis of the 2 degree target. He estimates that staying below 2 degrees is expensive compared to future financial damage measured in GDP.
Many political leaders have pointed out that developing countries need cheap energy to lift their citizens out of poverty. Any temperature limit must balance the cost of technologies that move away from fossil fuels with the ability of nations to improve the lives of their people.
Whatever the correct temperature target, its link to emissions has played a useful role in providing a common goal against which to measure progress. This is of course a complex problem.
We are in a competition between the cost and effectiveness of our efforts to reduce emissions, the global expansion of the human population of the Earth, the increased use of energy by developing countries and the increase in global average temperature. Meanwhile, the expected increase in extreme weather events has begun.
Patrick Love is a retired physicist who runs a small consulting firm in Tallahassee and can be reached at [email protected] This is a Greening Our Community article, an initiative of Sustainable Tallahassee. Learn more at SustainableTallahassee.org.
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