Weighing the implications of carbon milestone
Editor's note: On May 9, global atmospheric concentrations of carbon dioxide, as measured at Hawaii's Mauna Loa Observatory, reached 400 parts per million for the first time since accurate measurements started there in 1958. University of Illinois atmospheric sciences Professor Don Wuebbles, an expert in climate change, talked with UI News Bureau physical sciences editor Liz Ahlberg about this carbon milestone and its implications.
Q: What does this milestone mean (both in definition and implication)?
A: Before the Industrial Revolution, natural levels of carbon dioxide in the atmosphere averaged about 280 parts per million. In other words, CO2 made up about 0.028 percent of the volume of the atmosphere. Over the last few centuries, emissions from human activities, especially the burning of fossil fuels such as coal and oil, along with changes in land use, have increased the atmospheric concentration of CO2 to the current level.
Despite being such a small portion of the atmosphere, carbon dioxide and other heat-trapping gases (also called greenhouse gases) are extremely important to Earth's climate. Without these gases, this would be a frozen planet. The reason why heat-trapping gases such as CO2 have such an influence on Earth's climate is that they are largely transparent to the visible and ultraviolet energy emitted by the sun but are very strong absorbers of the infrared heat energy emitted from Earth's surface. They then radiate some of this heat back to the surface, effectively trapping the heat inside Earth's climate system and warming Earth's surface.
The problem is that the concentration of CO2 will not stop at 400 ppm but will continue to increase as a result of human emissions, at least until we greatly decrease those emissions. The atmospheric lifetime of CO2 is very long, so we are committing future generations to higher levels of CO2 and a different climate than we have seen in the past.
Q: What makes this a milestone? Can you put it in historical perspective for us?
A: The last time Earth saw CO2 levels as high as 400 ppm was more than 2 million years ago, during the Pliocene Epoch (about 2.6 to 5.3 million years ago). The difference now is how rapidly we are increasing the levels of CO2. Recent estimates suggest CO2 levels reached as high as 415 ppm during the Pliocene — and we will likely pass that level in the next decade. With that level of CO2, during the mid-Pliocene, global temperatures reached averages of 5 to 7 degrees Fahrenheit higher than today's temperatures and as much as 18 degrees warmer at the poles. Sea levels ranged from 15 to 130 feet higher than today. A recent paper in the journal Science discusses the much warmer polar regions during the Pliocene compared to today. The large heat capacity of the oceans will keep us from reaching such effects quickly, especially in terms of sea level rise, but the problem is that CO2 levels are still rising and could reach 500-800 ppm or more before the end of this century unless we greatly slow down the rate of increase.
Q: What can be done to reduce the carbon dioxide in the atmosphere? Or are we too late?
A: There are multiple paths forward in response to climate change. One choice is do nothing and try to deal with the consequences. However, a number of economic analyses have concluded that the costs from inaction would be much larger than the costs of action. Another choice is to significantly reduce the emissions of heat-trapping gases by changing the way that we use energy and transportation systems. Through using energy and transportation more efficiently and by switching to alternative sources of energy that reduce or eliminate the emissions of CO2 (and other heat-trapping gases and particles), we can significantly limit the effects on climate over the coming decades.
Increased efficiency in energy use is important, as is the increased use of energy technologies that do not produce CO2. For example, because about 28 percent of the energy used in the U.S. is used for transportation, changing the types of fuel that we use and driving more efficient vehicles is one obvious path forward. A large amount of energy in the U.S. is also used to heat and cool buildings, so changes in building design could dramatically reduce energy use. There are many pathways that can help prevent the largest of the potential impacts on humanity and ecosystems from climate change.
Q: What implications does this milestone have for policy?
A: Like the first explorers, we are entering uncharted territory — there has never been this much atmospheric CO2 in the human experience. This milestone is another indicator of the urgency for policies to be implemented to reduce human-related emissions of carbon dioxide, as well as those of other heat-trapping gases such as methane and nitrous oxide. Policies are needed that will prevent the worst of the changes in climate that will otherwise occur. Some further changes in climate are inevitable, but we can limit the impacts and prevent the largest changes in temperature and in severe weather that are likely without such actions.
Adaptation will also be necessary. Because impacts from the changing climate are already occurring and anticipated to increase at least in the short term, adaptation to the impacts of climate change will be required. Adaptation decisions range from being better prepared for extreme events such as floods and droughts, to identifying economic opportunities that come from investments in adaptation and mitigation strategies and technologies, to integrating considerations of new climate-related risks into city planning, public health and emergency preparedness, and ecosystem management.
Don Wuebbles is a professor in atmospheric sciences at the University of Illinois.