The Science of Climate Change
Climate change is a reality in our world today. As stewards of the planet, we have a responsibility to help reduce our impacts in our day-to-day lives. Fortunately, Energy Impact Illinois is here to help you save the planet and save money by reducing your energy use. Read below to learn more about the science of climate change and why it is important to start reducing our greenhouse gas impacts today!
Scientific Consensus on Climate Change
There is a strong scientific consensus that human-caused emissions in greenhouse gases (carbon dioxide, methane, nitrous oxide, halocarbons) are the primary driver of modern climate change. This consensus is supported by the Intergovernmental Panel on Climate Change (IPCC), the U.S. National Academy of Science, the National Academies (or equivalent bodies) of all major industrialized countries, the American Association for the Advancements of Science, the American Geophysical Union, the World Meteorological Organization, and dozens [http://en.wikipedia.org/wiki/Scientific_opinion_on_climate_change] of other scientific organizations. While plenty of questions remain about some of the details, there is virtually no disagreement [http://www.sciencemag.org/cgi/content/full/306/5702/1686] in the scientific literature over the fundamental facts that the Earth is warming and human activities are the primary driver of this warming over the past few decades.
Annual NASA GISTemp instrumental temperature data, 1850-2010. Image from Robert Rhode’s Global Warming Art and Wikipedia [http://en.wikipedia.org/wiki/File:Instrumental_Temperature_Record.png]. Temperature Anomaly is calculated relative to the 1951-1980 mean temperature.
Human-caused emissions of greenhouse gases have increased rapidly over the past 250 years since the industrial revolution led to the burning of fossil fuels to produce energy. These gases accumulate in the atmosphere, absorbing the reradiating heat back to the surface of the Earth and increasing the mean surface temperature. The image to the right shows the atmospheric concentration of the three major greenhouse gases over the past 10,000 years, with all three spiking dramatically over the past few centuries.
If greenhouse gas emissions continue unchecked, global temperatures are projected to increase between 2.0 and 11.5 degrees F over the next century, with the wide range of possible warming dependent on future emissions scenarios. The middle range of projected warming would result in a climate possibly warmer than any time in the past million years. It would likely result in [http://www.ipcc.ch/ipccreports/ar4-wg2.htm] widespread species extinction, sea level rise, more frequent extreme weather events and droughts, and other negative impacts.
Energy Flows and Climate Impacts
The vast majority of energy in the Earth’s climate system comes from the sun. Every year around 341 watts of solar energy falls on every square meter of the Earth’s surface. Of this incoming solar energy, 79 watts per square meter (Wm-2) is reflected away by clouds and small particles in the atmosphere and never reaches the Earth’s, and another 23 Wm-2 is reflected directly by the Earth’s surface. The remainder is absorbed by the atmosphere and the Earth’s surface and is reradiated in the form of heat (e.g. longwave radiation).
If the Earth had no atmosphere, and was only heated by the sun, it would be uninhabitable. Average temperatures would be a chilly 0 degrees F, with nighttime temperatures plummeting much lower. Thankfully, greenhouse gases in the Earth’s atmosphere trap heat radiated from the Earth’s surface before it can escape from space, reradiating a portion of it back to Earth.
The greenhouse gases in the Earth’s atmosphere keep our planet habitable, and regulate the Earth’s temperature. While levels of greenhouse gases in the atmosphere can change dramatically over hundreds of millions of years, they have remained relatively stable over at least the past million years (the period during which our species evolved). Atmospheric CO2 concentrations have fluctuated [http://www.globalwarmingart.com/wiki/Image:Ice_Age_Temperature_Rev_png] from a low of 180 parts per million (ppm) during ice ages to around 280 ppm during interglacial periods. Since the industrial revolution, however, the atmospheric concentration of carbon dioxide has increased rapidly from 280 ppm to 386 ppm.
Unsurprisingly, greenhouse gases make up the bulk of warming, with some additional contribution from low-level ozone (e.g. smog) that reduces the amount of sunlight reflected back to space.
Natural forces also affect the Earth’s climate. Solar output [http://www.yaleclimatemediaforum.org/2008/05/common-climate-misconceptions-solar-influences-on-global-temperature/] has increased slightly over the past century, though it has been largely static for the last thirty years. Periodic volcanic eruptions cause cooling by injecting large amounts of sulfate aerosols high in the atmosphere, though the effects of these fade after a few years due to the relatively short atmospheric lifetime of aerosols. The last major volcanic eruption that affected the Earth’s climate was that of Mt. Pinatubo in 1992.
The relationship between climate forcings such as greenhouse gases and aerosols and the actual change in the Earth’s temperature is complicated by a number of factors. Chief among them are various feedbacks and thermal inertia. The Earth is mostly covered by water, and it takes an enormous amount of energy to heat water. As greenhouse gas concentrations in the atmosphere increase, the oceans and ice sheets absorb much of the extra heat. It may take hundreds if not thousands of years for the deep oceans to heat up and the ice sheets to completely melt before the true equilibrium temperature associated with a certain atmospheric greenhouse gas concentration is reached. The interaction between forcings, feedbacks, and thermal inertia determine the Earth’s climate sensitivity, generally defined in terms of the amount of warming expected per doubling of carbon dioxide (or equivalent greenhouse gas) emissions.
Could Warming be Natural?
The Earth’s temperature has changed in the past, often dramatically. There are periods prior to the industrial revolution, such as the medieval warm period or the little ice age during which global temperatures warmed or cooled, albeit (to the best of our knowledge) not as dramatically as in recent years. Much more dramatic swings occurred during ice ages, where the Earth’s temperature oscillated by more than 10 degrees F and massive ice sheets grew and retreated. So if natural factors can cause the Earth’s temperature to change in the past, how do we know that current warming could not be natural as well?
It turns out that there are a relatively small set of natural forcings that can act to change the earth’s climate over time. The major culprit for the dramatic ice ages over the last few hundred thousand years are changes in the Earth’s orbit called Milankovitch Cycles [http://en.wikipedia.org/wiki/Milankovitch_cycles]. These occur slowly over tens and hundreds of thousands of years, and we can accurately measure and model them with various instrumental measurements. These measurements clearly show that there are no changes to the Earth’s orbit that are causing modern warming.
Another major natural forcing is the Sun, thought to be responsible for the little ice age and (possibly) the medieval warm period. However, modern satellite measurements of total solar irradiation (as well as secondary sun-moderated factors like galactic cosmic rays) show that while the Earth has been warming over the past 50 years, incoming solar radiation has actually decreased slightly. Solar factors may have played a role in warming during the early part of the century (1900 to 1950), but is very unlikely to have been the cause of the rapid warming from 1970 to present.
If the rapid rise in concentrations of atmospheric carbon dioxide and other greenhouse gases does indeed explain the majority of modern warming, as the physics of radiative transfer would suggest, how do we know that rising carbon dioxide concentrations are due to humans? It turns out that rising atmospheric levels of carbon dioxide (which have been relatively stable for at least the past million years, if not considerably longer) perfectly mirrors measured emissions of carbon dioxide from burning fossil fuels and land-use changes like deforestation. Indeed, about half of emitted carbon dioxide each year remains in the atmosphere, with the remaining half being absorbed by vegetation and the ocean. We also know from analyzing the isotopic signature [http://www.realclimate.org/index.php/archives/2004/12/how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human-activities-updated/] of the atmospheric carbon dioxide that the majority of it comes from long-buried fossil fuels rather than inorganic sources or current vegetation.
General circulation models of the Earth’s climate can take our best knowledge of various natural and human-caused forcings to determine how much of the temperature change over the last century is due to each factor. The graph below shows climate model outputs for each continent (as well as the world as a whole) for runs in which human-caused forcings were excluded (blue bar) and included (pink bar), as well as observations of actual temperatures (the black lines) in each location. It shows that observed temperatures after 1970 are well outside the range predicted by including only natural forcings, and are very likely driven by human emissions of greenhouse gases.
The evidence clearly shows that climate change is occurring, and it is occurring because of greenhouse gases emitted by humans. Energy Impact Illinois is here to help you reduce your own impact while saving money through lower energy bills.