Greenhouse Effect: Knowledge

The greenhouse effect describes the natural process in which certain gases in the earth’s atmosphere trap and redirect heat back to the earth’s surface that would otherwise be sent out to space. The earth’s atmosphere acts like a greenhouse, where heat is let in but is not allowed to escape. The gases that trap this heat are greenhouse gases (GHGs), and their concentration in the atmosphere has a significant effect on the earth’s climate. Without the natural greenhouse effect, the earth’s average temperature would be about 0°F (-18ºC) instead of the current 57° F (14ºC) [1].
 

The greenhouse effect is not controversial; it’s well-documented and is a basic concept in earth’s atmosphere. The main concern is whether human activities (e.g., deforestation and fossil fuel burning) can significantly enhance the effect to cause additional warming [1]. A 2007 U.N. Intergovernmental Panel on Climate Change (IPCC) report projected the earth would warm 3.2-7.2°F over the next century. How much it warms mainly depends on GHG and aerosol emissions from humans, and how key earth processes respond to these emissions [11].

The sun is constantly showering the earth with light (also called energy, or solar radiation). When this energy hits the earth, the planet’s surface warms and emits some energy back into space to stay cool. GHGs are sensitive to this new energy type emitted by the earth, and they are able to absorb and emit some of it back to the earth’s surface. In this way, the atmosphere acts as a blanket for the earth’s surface, constantly keeping it warm.  As GHG concentrations increase, the blanket traps heat better, and the earth’s surface will warm as a result.

 

Carbon dioxide (CO2) is the most influential GHG, and its atmospheric increase since 1850 is attributed to anthropogenic activity.  This gas is the main cause for the significant warming that is projected to occur globally by the year 2100.

The “greenhouse effect” was first documented in the 1800s by the British scientist John Tyndall. He found that certain gases absorbed heat very effectively and theorized that without them, the earth’s surface would freeze [3].

 

Svante Arrhenius, a significant contributor to physical chemistry, was the first to suggest that humans would increase the Earth’s temperature through the release of GHGs  in 1895 [4, 5].  After Arrhenius, studies connecting the greenhouse effect to humans were nonexistent for almost half a decade, due to equipment and historical record limitations. CO2 levels in the atmosphere, for example, were not directly recorded until the Keeling Curve in 1958 [8]. Other techniques for approximating past climate conditions, such as ocean and ice cores, were also not developed until the mid- to late-20th century [12].

 

Today, climatologists can forecast the earth’s climate to the end of the century using global climate models, which are able to simulate the evolution of the climate system based on today’s ocean, land, and atmosphere. These models have improved extensively since their debut in the 1980s, and modeling centers employ supercomputers to run simulations. Even on these supercomputers, the models are so computationally expensive that each run takes on the order of a month to complete [10].
 

Climate models have been able to account for new factors, such as vegetation and volcanic activity, over the last few decades. The above image shows the progression of climate models and IPCC report projections since the 1970s (FAR-- 1990, SAR-- 1995, TAR--2007, AR4--2011)   [6].  

Water vapor is both the most abundant and the most effective GHG [13]. By volume, water vapor accounts for about 90% of GHGs in the atmosphere [18]. Despite its potency and abundance, water vapor is not the root of long-term temperature variability. Instead, it mainly amplifies -- and doesn’t cause or force -- long-term temperature change. In this way, water vapor is often referred to as a “radiative feedback” of the climate system, and not a “radiative forcing.”

 

Water vapor is well-regulated by the earth system, and although it’s the strongest GHG, it typically exists in the atmosphere for 10 days before it is rained or snowed out. CO2, by contrast, takes decades to leave the atmosphere. Therefore, CO2 and other long-lived GHGs are seen as responsible for the radiative forcing known to cause global warming.

 

Due to water vapor’s short life time, the amount in the atmosphere is related to the earth’s temperature:  more water vapor can be held in the atmosphere when the earth is hotter. Since water vapor is a GHG, any additional vapor that evaporates as a result of a warmer climate will cause temperatures to rise even further. Through this process, warming due to higher CO2 concentrations can be amplified by up to two times [13]. Because water vapor also is key in cloud formation and other climate-related processes, the extent that it will enhance future warming is still poorly understood, and cloud processes are an active area of research in model improvement [15].

CO2 is the root of the current warming trend. The IPCC projects that CO2 is nearly four times more effective in causing warming than the second-most effective GHG [19]. For this reason, GHG regulation efforts usually focus on CO2. Although carbon dioxide is naturally absorbed and produced, people have altered the natural balance by vigorously burning coal, oil, natural gas, and wood.

 

Since the industrial revolution that began around 1850, the amount of CO2 in the atmosphere has increased by 40% from 280 ppm (parts per million, a unit to measure atmospheric gas concentrations) to 390 ppm. It is currently increasing at a rate of 2 ppm per year [20].

CFCs have no natural source in the earth. They are a man-made compound created for use as refrigerants, aerosol propellants, and cleaning solvents. Due to a few major discoveries in the 1970s, CFC concentrations are known to have increased significantly since their use began in the first half of the 20th century. It was soon shown CFCs are extremely effective in destroying stratospheric ozone, a compound that protects the earth from harmful UV radiation. As a result, an international agreement took effect in the 1970s to phase out the use of CFCs. Due to the agreement’s success, CFC concentrations are expected to stabilize and then decline in the early 21st century.

Although the greenhouse effect is a natural phenomenon, humans are warming the planet by adding more greenhouse gases. All else equal, warming will occur when more greenhouse gases are released. The exact extent to which the earth will warm is not certain because it is not understood how other earth processes will react to increased CO2. As a result, future temperatures are often given in ranges to reflect this uncertainty.

 

Human activity has been increasing the concentration of GHGs in the atmosphere through the combustion of coal, oil, and gas, and scientists track steady annual increases each year. 

 

According to the IPCC, by the end of the 21st Century, carbon dioxide concentrations will likely be 75-350% above pre-industria values. By 2100, this implies a temperature increase of about 2.5-10°F. This is significant, because at the end of the last ice age, average temperatures were only 5-9°F cooler than today [16].  It is also just an average; extreme weather events (heat waves, floods, etc.) are also expected to intensify with global warming [22].

http://www.ncdc.noaa.gov/oa/climate/globalwarming.html

http://www.ucar.edu/learn/1_3_1.htm

http://earthobservatory.nasa.gov/Features/Tyndall/

http://www.chemheritage.org/discover/chemistry-in-history/themes/electrochemistry/arrhenius.aspx

http://earthobservatory.nasa.gov/Features/Arrhenius/

http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter1.pdf

http://www.giss.nasa.gov/research/features/200711_temptracker/

http://airs.jpl.nasa.gov/story_archive/Measuring_CO2_from_Space/History_CO2_Measurements/

http://www.skepticalscience.com/water-vapor-greenhouse-gas.htm

http://www.wesjones.com/climate2.htm

http://www.epa.gov/climatechange/science/futuretc.html

http://www.aip.org/history/climate/cycles.htm

http://www.nasa.gov/topics/earth/features/vapor_warming.html

http://earthobservatory.nasa.gov/Features/WaterVapor/water_vapor2.php

http://www.ncdc.noaa.gov/oa/climate/gases.html

http://climate.nasa.gov/effects/

http://e360.yale.edu/feature/can_we_trust_climate_models_increasingly_the_answer_is_yes/2360/

http://www.realclimate.org/index.php/archives/2005/04/water-vapour-feedback-or-forcing/

http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-3.html#2-3-1

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/grnhse.html

http://earthobservatory.nasa.gov/Features/EnergyBalance/page6.php

http://www.ipcc-wg2.gov/SREX/