Some 12,000 years ago, at the start of the current geologic epoch called the Holocene, the average global temperature was roughly 6 degrees Centigrade lower than the average temperature from then to now (i.e., 11 degrees Fahrenheit colder).
Consequently, global average sea level was 100 feet lower due to the increased amount of water captured in ice and snow.
Observations collected over the last 200 years indicate that since the 1800s the global average temperature has increased about 1.1 degrees Centigrade. Global average temperature is a measure of the Earth’s surface temperature, i.e., the first few meters of the atmosphere and the oceans.
The global average temperature is based on measurements from some 10,000 land stations, and numerous measurements at sea by ships and robotic floats. While there has been year to year variability the global temperature has been steadily warming. The last 10 years have been the hottest years on record since measurements began in the 1800’s.
The idea of limiting global average warming to 2 degrees C measured above the average from 1850 to 1900 was first discussed in the early 1990’s to create a target that the world’s nations could work together to achieve.
This limit was first endorsed by the EU and then adopted by the global community in the 2009 Copenhagen accord on climate. It was further strengthened at the 2015 Paris Conference on Climate Change where 195 countries agreed to report and reduce emissions that cause warming.
Greenhouse gas and temperature
To support the 2-degree limit, climate scientists developed models that connected greenhouse gas concentrations to global temperature. About half the emissions from human activities are absorbed by the oceans and the terrestrial biosphere.
The remaining atmospheric concentrations of greenhouse gases impede the escape of heat energy from the Earth’s surface to space.
Relatively simple models that balance the energy received from the sun with the amount that escapes to space using greenhouse gas concentrations predict near surface warming. It is currently estimated that doubling the preindustrial greenhouse gas concentrations of 280 ppm (parts per million) would raise the global temperature by 2.7 to 3.4 degrees C. The concentration today is about 414 ppm.
Although it is hard to be certain about the amount of future greenhouse gas emissions, a consensus estimate suggests that we might 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 increase 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 circulation of energy from the equator to the poles is how weather events are powered. The increase in global average temperature measures an increase in the energy of the fluids that circulate on the Earth’s surface.
A limit like 2 degrees C is small compared to the fluctuations in temperature we observe during a year. If you were to record daily temperature over a year you would assemble a distribution of temperatures. You might experience one day above 95 degrees F each year. A 2-degree C warming would shift your distribution to higher temperatures by 3.6 degrees F.
After the shift some of the daily temperatures between 90- and 95-degrees F move to between 95- and 100-degrees F. Thus those higher temperatures, once considered extreme and infrequent, become more common. The threat of climate change is not so much that the world gets a little warmer but rather that the probability of extreme temperatures becomes more likely.
Uncommon temperature extremes
Extreme temperatures accelerate a decrease in outdoor worker productivity and make parts of the Earth difficult places to live. The summer temperatures in England reaching record highs above 104 degrees F is one of many examples of uncommon temperatures in 2022.
Sea level rising is caused by both the expansion of hotter water and the melting of ice and snow resulting in tidal flooding and increased storm surge during coastal storms. Flooding in Miami during tidal events has become common necessitating significant infrastructure changes to the city.
Higher air temperatures capturing more moisture lead to increased precipitation that cause flooding. Some 10 to 15% of Pakistan was recently underwater during 3 months of record rains.
Hurricanes can, and have, intensified rapidly due to a warmer ocean surface. The intensification leads to increased storm wind speeds and increased moisture content. Hurricane Harvey in Houston, Texas, and Florence in North Carolina carried some 20% more moisture to shore due to warm surface waters. Heavy rainfall is the leading cause of damage from tropical storms.
The Nobel-prize winning economist William Nordhaus pioneered a cost benefit analysis of the 2-degree target. He estimates that staying below 2 degrees is expensive relative to future financial damage measured in GDP.
Many political leaders have pointed out that developing nations require 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 their people’s lives.
Whatever the correct temperature target, its link to emissions has served the useful role of providing a common goal to measure progress against. This is of course a complex problem.
We are in a contest between the cost and effectiveness of our emissions reduction efforts, the global expansion of Earth’s human population, the increased use of energy from developing countries, and the increasing global average temperature. Meanwhile the expected increase in extreme weather events has begun.
Patrick Love is a retired physicist who runs a small consulting company in Tallahassee and can be reached at email@example.com. This is a “Greening Our Community” article, an initiative of Sustainable Tallahassee. Learn more at SustainableTallahassee.org.
Never miss a story: Subscribe to the Tallahassee Democrat using the link at the top of the page.
This article originally appeared on Tallahassee Democrat: 2 degrees of Centigrade explained: What warming temperatures mean