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Global Warming, more commonly referred to as climate change, is defined as "An increase in the average temperature of the Earth's atmosphere, especially a sustained increase great enough to cause changes in the global climate." (1). A climate forcing is one that changes the global energy balance between incoming, or solar based, and outgoing heat from the surface of the Earth. There are a number of natural climate forcings that can upset the energy balance of the planet. Some of these include changes in solar irradiance, surface albedo, global dimming and global brightening, atmospheric composition (greenhouse gases), variations in ocean circulation, lapse rate and so on.

Lets begin with solar irradiance or the energy that strikes the Earth that originates from the Sun. The Sun is what provides the planet with all it's energy and heat and has been the main natural driver of Earth's climate since it's birth. Graphs depicting solar output and global temperature variations since measurements began show a close resemblance between the two. However since the late 70s the Sun's output has fallen while the temperatures have continued to rise (See image and source).

The energy, or radiation, that the Sun sends to the Earth is in the ultraviolet, visible and infrared regions of the electromagnetic spectrum. When that radiation hits the atmosphere some of it is redirected via a process known as dimming, caused by sulfate aerosols in the atmosphere, or surfaces with high albedo, such as snow. That radiation that does make it to the surface is absorbed and re-emitted according to Kirchoff's Law of Thermal Radiation, which states that any object at some non-zero temperature radiates electromagnetic energy. According to Planck's Radiation Law the emitted radiation varies in frequency and wavelength dependent on the temperature of the object. The Earth, being much cooler than the Sun, emits radiation at a longer wavelength.

This radiation travels upward through the atmosphere on it's way back to space. However, there are several types of gases that can absorb specific wavelengths of radiation impeding that path. These are called greenhouse gases. When heat radiation strikes one of these gases the radiation is absorbed, the molecule gains kinetic energy via molecular vibrations, which are restricted via the number of atoms in a molecule, and re-releases the energy in all directions, some of it back toward the ground where that energy is absorbed and the cycle begins again. This process keeps temperatures approximately 33° warmer than if these gases were not present (2). A process known as the Greenhouse Effect. It is an essential part of keeping our planet hospitable and helps in maintaining life.

The theory of global warming, or the increase in temperature due to the concentration of greenhouse gases in the atmosphere, was first proposed by Swedish physicist and chemist Svante Arrhenius (1859-1927) in 1896. He proposed a relation between atmospheric carbon dioxide and global temperature change that made use of Joseph Fourier's (1768-1830) Greenhouse Effect, discovered in 1826, and that a doubling of CO₂ would amount to a 5° increase in temperatures (1). It wouldn't be until 1956 when Gilbert Plass (1920-2004) summarized the results and developed an early model of atmospheric radiative transfer which, contrary to his predecessors, he saw as a problem (2). The first measured evidence of an increase in CO₂ was the so-called Keeling Curve which is based on continuous measurements at the Mauna Loa Observatory under the supervision of Charles Keeling (3). Keelings measurements are the longest continuous measurements of carbon dioxide in the world and have shown that between 1958 and 2008 atmospheric CO₂ concentration has increased from 315 parts per million (ppm) to over 380ppm (4). In 1967 the first computer model of CO₂ concentration found that a doubling of CO₂ would cause temperatures to rise roughly 2.3°C (5).

Carbon Dioxide is composed of a number of different isotopes, 2 of which are stable. Carbon-12, of which approximately 98.9% of the carbon in the atmosphere is composed, Carbon-13, which covers approximately 1.1%, and Carbon-14, a radioactive isotope that occurs in trace amounts are the three most abundant. For this comparison we will be using Carbon-12 and Carbon-13. During photosynthesis plants have the ability to descriminate against Carbon-13 due to differences in chemical and physical properties imparted by the difference in mass (6), the effect of which is roughly a 2% lower Carbon-13 ratio than that of the atmosphere. Therefor, if the increased concentration of atmospheric CO₂ is primarily due to fossil fuel combustion, which originates from plant material, we should see the decrease in the isotopic ratio of the atmosphere. The data, as measured at various stations around the world, shows a change of about 0.15% in the last 150 years (7,8).

Since 1970 satellites have been able to measure temperatures via measurements of radiances in various wavelength bands. Both the IRIS satellite, launched by NASA in 1970, and the IMG satellite, launched by Japan in 1996, measure these observances. When both measurements are compared we see that, as the years progress, more radiation is being absorbed by the planets atmosphere (9). Specifically, more radiation is being absorbed at wavelengths associated with CO₂ and CH₄ absorption as well as the absorption wavelengths of various other minor greenhouse gasses (10). Other factors, such as solar irradiance (11) and Milankovitch cycles, or the collective effects of changes in the Earths orbit and axial tilt, have been ruled out to be the main cause of the current increase in temperatures. (Update) This study was further updated in 2007 to include AIRS (Automatic Infrared Sounder) data. The new data supports the conclusions done in the earlier work. The abstract and journal can be found here.

Further evidences of an increasing greenhouse effect exist in both measurements of downward longwave, or infrared, radiation and cooling stratospheric temperature measurements. Radiation fluxes of downward longwave radiation have been observed and measured at thousands of stations around the world from as far back as 1973. Comparative measurements show an increase at an average 2.2 W/m^-2 per decade between 1973 and 2008 (12). Specifically, measurements at wavelengths attributable to anthropogenic emissions show an increased forcing (13). This is causing more heat to be trapped in the troposphere while allowing less of it to extend to the stratosphere. Though the lapse rate within the stratosphere is opposite that of the troposphere, that being it warms with increasing height as opposed to cooling with increasing height, the effects of an increase in greenhouse gases causes a greater amount of cooling with increasing altitude. Observations of stratospheric temperature trends have been regularly assessed as part of the WMO/UNEP Scientific Assessments on Ozone Depletion. The most recent of which shows a 0.5K - 1K cooling per decade for the lower stratosphere and more than 2K in the upper stratosphere and lower mesosphere (14) and, as a result, increased altitude of the tropopause, or the boundary between the troposphere and the stratosphere (15).

In 1988 a group, that would become known as the Intergovernmental Panel on Climate Change (IPCC), was formed by The United Nations Environment Programme (UNEP) and The World Meteorological Organization (WMO) to provide the world with a clear scientific view on the current state of climate change and its potential environmental and socio-economic consequences. The groups goal is to review and assess the most recent scientific, technical and socio-economic information produced worldwide relevant to the understanding of climate change. It does not conduct any research nor does it monitor climate related data or parameters (16). The IPCC has recently come under the microscope with claims of non-peer reviewed literature and other sources being used in their reports (17). (Update) On May 7, 2010 over 250 scientists, including 11 Nobel Laureates, signed a statement concerning climate change and the integrity of science after concluding that the political battle, over such things as the hacked emails, is eroding confidence in their work. The statement can be found here.

Probably the most discussed climate changing impact a warming planet has on the environment is the decrease of glacial mass and, consequently, a rise in sea level and desalinization of the oceans. Measurements from the World Glacier Monitoring Service (WGMS) (1) and the GRACE satellite monitoring system (2) have shown substantial losses in most of the glaciers of the world. Increased melting of glaciers has contributed to sea level rise, as measured by tide gauge measurements and satellite altimeter measurements, at a rate of 3.2 ± 0.4mm / year since 1992 (3). A study in 2003 by Ruth Curry et al found ocean salinity has been declining at both poleward ends of a region between 50°S and 60°N (4) corresponding with glacial melt.

One of the major effects of a warming climate is a rise in ocean temperatures leading to such things as mass coral bleaching and species migration, both of which are currently being seen in ocean ecosystems. Coral bleaching occurs when corals are stressed by such things as over fishing, changes in water chemistry, increased sedimentation, changes in salinity and changes in ocean temperature. As a result of these stressors the densities or concentration of photosynthetic pigments of zooxanthellae, autotrophic protozoa that provide the host with energy and nutrients, decline. The corals lose their main source of nutrition and, if bleaching persists for an extended period of time, the coral host may die (5). Since 1979 there have been dozens of reports of coral bleaching due to elevated sea surface temperatures while during the previous century there had been 3 (6).

Climate change is affecting plants, wild animals and birds and allowing various forms of bacteria and viruses to move poleward. In springtime many plant species flower earlier than they did a century ago. Migrating birds have changed their migration schedules based on the rising temperatures, they migrate earlier and stay longer at their migratory habitat (7). Recent studies show that phenological events related to an increase in temperatures amount to an advancement of 5.1 days per decade on a global scale (8,9). An increase in vector-borne diseases, or diseases that transfer via infected organisms, is expected due to both environmentally induced migrations and warmer temperatures (10). An incident of this was observed in the 1990s when local transmissions of malaria, a common vector-borne disease in warmer temperatures, appeared in New York and New Jersey during a pattern of exceptionally hot and humid weather (11). Yet another example of vector-borne illnesses moving northward is the first appearance of the West Nile virus in the New York area in 1999. Since 2000 there have been 254 cases of the disease in the State of New York with 26 deaths (12). Other types of diseases that are known to have spread are chagas disease, onchocerciasis, dracunculiasis, lymphatic filariasis, dengue fever, schistosomiasis, leptospirosis, strongyloidiasis, cholera, trypanosomiasis, leishmaniasis, neurocysticercosis, trichuriasis, pneumonic plague, hantavirus and viral encephalitis.

Polar bears have become the poster child for both arguments concerning a changing climate. Environmentalists show sea ice and polar bear populations decreasing while their opponents state that polar bear numbers are actually increasing. Believe it or not, both arguments are correct. The Multilateral Conservation of Polar Bears Agreement was signed by 5 circumpolar countries, Canada, Denmark, Norway, the USSR and the United States, in Oslo on November 15, 1973 and was meant to limit killing, hunting and the capturing of polar bears except for limited purposes and by limited methods (13). Since this took effect polar bear numbers have rebounded. However, the effects of a warming climate are beginning to take a toll. Today the worldwide polar bear population ranges from between 20,000 to 25,000 in 19 distinct populations and was listed as a threatened species under the Endangered Species Act of 1973 on May 14, 2008 (14). A 2006 study conducted by the US Geological Survey of 4 arctic regions found that populations were stable in 2 of the regions while numbers were declining in the other two (15). The 15th meeting of the Polar Bear Specialist Group (PBSG) found, with the latest information available, that of the 19 total populations of polar bears at least 3 are stable while 8 are declining and only 1 is growing. The rest didn't have enough data to make an assessment (16).

Another impact of a larger CO₂ concentration in the atmosphere is an increased uptake of CO₂ by the ocean. The ocean, as is widely known, is the largest carbon sink in the world. Of course as the carbon dioxide from the atmosphere is increased it creates more of a stress on the oceans and it's chemical processes. An increased uptake of CO₂ results in what is known as ocean acidification and a decrease in ocean pH and carbonate ion concentrations. Experimental evidence suggests that as a result corals and some plankton that use calcium carbonate for their exoskeleton will have difficulty maintaining them (17).

Yet another impact of a warming of the atmosphere is an increase in hurricane severity (Also known as a cyclone or typhoon). A hurricane forms when warm, moist air rises and, when contact with cooler air is made, condenses to form thunderstorms. Wind speeds then increase due to the Coriolis Effect and, with energy provided by warm ocean waters and the latent heat from condensation, form a tropical depression. If this escalates it eventually forms a hurricane and continues to gain in strength due to the cycling of warm air at the bottom and cooling air at the top. This continues until conditions deny the storm the warm, moist air it requires such as when landfall is made or when the storm reaches an area of cooler ocean temperatures (18). Though it is still unclear whether hurricane frequency relates to atmospheric warming due to improved instrument sensitivity, there is evidence showing that hurricane severity increases with a warmer climate (See Table 1).

Another sign of planetary warming is the expansion of the tropical Hadley Cell. Warmer tropical air rises and flows poleward in both hemispheres where it cools. sinks, and flows back to the tropics generating an enclosed circulation in each hemisphere. This circulation is related to phenomena such as the trade winds, tropical rain belts, subtropical deserts and jet streams. Several lines of evidence show that over the past few decades the tropical Hadley cell has expanded, the effects of which, while not well understood, could lead to substantial changes in the global climate system (20).

"The missing hotspot"
The hotspot that skeptics use as ammo for proof that anthropogenic global warming does not exist isn't based on human induced warming by CO₂ gases at all. Rather it is a consequence of any type of warming. The tropical hotspot is projected by computer models due to something known as the moist (or saturated) adiabatic lapse rate. The lapse rate is defined as the rate of decrease in temperature of a mass of air with increasing altitude. The moist adiabatic lapse rate applies to air parcels at their dew point. A parcel of air at it's dew point cools much more slowly than a parcel of air that is not. The rate of temperature decrease of a dry air parcel is 9.8°C per 1000 meters while the moist adiabatic lapse rate, which depends on specific humdity, is about 3 times lower at a temperature of 35°C (1). The projected hotspot is the consequence of a saturated air parcel rising through the atmosphere until it becomes cool enough to condense into vapour, releasing it's latent heat. The reason why a tropical hotspot has not been detected yet is most likely data problems with radiosonde and satellite observations and measurements (2).

"Negative cloud feedbacks..."
Many, some even in the scientific community, question the feedbacks related to cloud variability. Clouds can act both as a negative feedback, or one that deflects solar radiation before striking the surface of the planet, or positive feedback, or one that traps more heat near the surface of the planet. Roy Spencer, who served as Senior Scientist for Climate Studies at NASA's Marshall Space Flight Center, is one of those who believe that cloud feedbacks with increasing temperatures would result in a negative feedback and postulated a lower climate sensitivity than the current consensus (3). However recent studies show a positive low-level cloud feedback related to an increase in greenhouse gases due to a reduction in cloud cover (4).

"CO₂ was higher during the ice ages!"
Though scientists are relatively certain that atmospheric CO₂ levels are one of the major drivers of climate change, there still remain a few apparent mismatches in the geological record, one of them being the late Ordovician glaciation. Looking past the weaker solar output, roughly -4.5% compared to todays, we see that CO₂ concentration stood at 14x what it is today yet this was a time of extreme glaciation. Why did this occur if CO₂ acts as a greenhouse gas? This was due to a number of factors, one of which was the position of the continents at the time, the specific point in that particular Milankovitch cycle, or the axial tilt of the Earth (5), and the decrease of CO₂ levels due to a combination of subduction of the Atlantic oceanic crust, chemical weathering and uptake of the resulting calcium carbonate by marine organisms for their shells (6). Another study found that if all the forcings and feedbacks where taken into account it would have been enough to drop the temperatures low enough for glaciation to occur (7).

"They deleted all the RAW data!"
While the original RAW data was deleted copies of all data sets still exist elsewhere and are freely available online.

• GHCN: Global Historical Climate Network - ftp://ftp.ncdc.noaa.gov/pub/data/ghcn/v2
• USHCN: US Historical Climate Network - ftp://ftp.ncdc.noaa.gov/pub/data/ushcn/
• World Monthly Surface Station Climatology - http://dss.ucar.edu/datasets/ds570.0/
• Antarctic weather stations - http://www.antarctica.ac.uk/met/READER/
• European weather stations - http://eca.knmi.nl/
• Satellite feeds - http://mirador.gsfc.nasa.gov/cgi-bin/mirador/presentNavigation.pl?tree=project&project=SORCE, http://amsu.cira.colostate.edu/
• World Glacier Monitoring Service - http://www.geo.unizh.ch/wgms/dataexp.html
• International Comprehensive Ocean/Atmosphere Data Set (ICOADS) - http://icoads.noaa.gov/

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