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Are any of concerned about global warming?   What is it actually?   Why do scientists make so much of it?   Should we be concerned?

Hopefully I will be able to share a light on this topic that has become a hot-button topic and that  for those out there will understand  global warming a bit better.   It has been noted that environmental organizations and public figures have emphasized changes in the climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions.

There are different views over what the appropriate policy response to climate change should be. These competing views weigh the benefits of limiting emissions of greenhouse gases against the costs. In general, it seems likely that climate change will impose greater damages and risks in poorer regions.

Global warming refers to an unequivocal and continuing rise in the average temperature of Earth’s climate system.   Since 1971, 90% of the warming has occurred in the oceans. Despite the oceans’ dominant role in energy storage, the term “global warming” is also used to refer to increases in average temperature of the air and sea at Earth’s surface. Since the early 20th century, the global air and sea surface temperature has increased about 0.8 °C (1.4 °F), with about two-thirds of the increase occurring since 1980.   Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.    Scientific understanding of the cause of global warming has been increasing. It has been reported that most of global warming was being caused by increasing concentrations of greenhouse gases produced by human activities.   In 2010 that was recognized by the national science academies of all major industrialized nations. The largest driver of global warming is carbon dioxide emissions from fossil fuel combustion, cement production and land use changes such as deforestation.  Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes.    Future climate change and associated impacts will vary from region to region around the globe. The effects of an increase in global temperature include a rise in sea levels, and a change in the amount and pattern of precipitation, as well as a probable expansion of subtropical deserts.  Warming is expected to be strongest in the Arctic, with the continuing retreat of glaciers, permafrost, and sea ice.  Other likely effects of the warming include more frequent extreme weather events including heat waves, droughts and heavy rainfall; ocean acidification; and species extinctions due to shifting temperature regimes. Effects significant to humans include the threat to food security from decreasing crop yields and the loss of habitat from inundation.

Responses to global warming include mitigation by emissions reduction, adaption to its effects, and possible future geoengineering.  The warming that is evident in the instrumental temperature record is consistent with a wide range of observations, as documented by many independent scientific groups.  Examples include sea level rise, widespread melting of snow and ice, increased heat content of the oceans, increased humidity, and the earlier timing of spring events, e.g., the flowering of plants.  The probability that these changes could have occurred by chance is virtually zero.

It has been observed that temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures.   Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation. The northern hemisphere is also naturally warmer than the southern hemisphere mainly because of meridional heat transport in the oceans which has a differential of about 0.9 petawatts northwards, with an additional contribution from the albedo differences between the polar regions. Since the beginning of industrialization the interhemispheric temperature difference has increased due to melting of sea ice and snow in the North. Average arctic temperatures have been increasing at almost twice the rate of the rest of the world in the past 100 years; however arctic temperatures are also highly variable. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.     Monthly CO2 measurements display seasonal oscillations in an upward trend; each year’s maximum occurs during the Northern Hemisphere’s late spring, and declines during its growing season as plants remove some atmospheric CO2.The climate system can respond to changes in external forcing’s. External forcings can “push” the climate in the direction of warming or cooling. Examples of external forcings include changes in atmospheric composition (increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations of the Earth’s orbit, around the Sun. Orbital cycles vary slowly over tens of thousands of years and at present are in an overall cooling trend which would be expected to lead towards an ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures.   The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in a planet’s atmosphere warm its lower atmosphere and surface.

Emission scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological, technical, and natural developments. In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced. Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century. Emission scenarios, combined with modelling of the carbon cycle, have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future. The popular media and the public often confuse global warming with ozone depletion such as the destruction of stratospheric ozone by chlorofluorocarbons. Although there are a few areas of linkage, the relationship between the two is not strong. Reduced stratospheric ozone has had a slight cooling influence on surface temperatures, while increased tropospheric ozone has had a somewhat larger warming effect.

Global dimming:  a gradual reduction in the amount of global direct irradiance at the Earth’s surface.  The main cause of this dimming is particulates produced by volcanoes and human made pollutants, which exerts a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion – CO2 and aerosols – have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO2 greenhouse gases such as methane. Radiative forcing due to particulates is temporally limited due to wet deposition which causes them to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in particulate concentrations will only delay climate changes due to carbon dioxide.  Black carbon is second only to carbon dioxide for its contribution to global warming.  In addition to their direct effect by scattering and absorbing solar radiation, particulates have indirect effects on the Earth’s radiation.  Sulfates act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets, known as the Twomey effect.  This effect also causes droplets to be of more uniform size, which reduces growth of rain drops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect.   Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of particulates represent the largest uncertainty in radiative forcing.

Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot directly absorbs solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.  When deposited, especially on glaciers or on ice in arctic regions, the lower surface can also directly heat the surface.   The influences of particulates, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extra tropics and southern hemisphere.  Contribution of natural factors and human activities to radiative forcing of climate change.

Depletion of the ozone layer, by chemical refrigerants has also resulted in a strong cooling effect in the stratosphere. If the sun were responsible for observed warming, warming of both the troposphere and stratosphere would be expected.    The climate system includes a range of feedbacks, which alter the response of the system to changes in external forcings. Positive feedbacks increase the response of the climate system to an initial forcing, while negative feedbacks reduce the response of the climate system to an initial forcing.    There are a range of feedbacks in the climate system, including water vapor, changes in how snow and ice cover affect how much the Earth’s surface absorbs or reflects incoming sunlight, clouds, and changes in the Earth’s carbon cycle.  The main negative feedback is the energy which the Earth’s surface radiates into space as infrared radiation.   Feedbacks are an important factor in determining the sensitivity of the climate system to increased atmospheric greenhouse gas concentrations.   Other factors being equal, a higher climate sensitivity which means that more warming will occur for a given increase in greenhouse gas forcing.   Uncertainty over the effect of feedbacks is a major reason why different climate models project different magnitudes of warming for a given forcing scenario. More research is needed to understand the role of clouds and carbon cycle feedbacks in climate projections.

Global warming has been detected in a number of natural systems. Some of these changes are described in the section on observed temperature changes, e.g., sea level rise and widespread decreases in snow and ice extent.    Anthropogenic forcing has likely contributed to some of the observed changes, including sea level rise, changes in climate extremes, declines in Artic Sea ice extent, and to glacier retreat.  Widespread coastal flooding would be expected if several degrees of warming are sustained for millennia. Melting of the Greenland ice sheet could contribute an additional 4 to 7.5 m over many thousands of years. Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean.   During the 21st century, glaciers and snow cover are projected to continue their widespread retreat. Projections of declines in Arctic sea ice vary. Recent projections suggest that Arctic summers could be ice-free (defined as ice extent less than 1 million square km) as early as 2025-2030.

In terrestrial ecosystems, the earlier timing of spring events, and pole ward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs.   It is expected that most ecosystems will be affected by higher atmospheric CO2 levels, combined with higher global temperatures. Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.

Climate change could result in global, large-scale changes in natural and social systems.  Two examples are ocean acidification caused by increased atmospheric concentrations of carbon dioxide, and the long-term melting of ice sheets, which contributes to sea level rise.  Some large-scale changes could occur abruptly, i.e., over a short time period, and might also be irreversible.  An example of abrupt climate change is the rapid release of methane and carbon dioxide from permafrost, which would lead to amplified global warming. Scientific understanding of abrupt climate change is generally poor. However, the probability of abrupt changes appears to be very low. Factors that may increase the probability of abrupt climate change include higher magnitudes of global warming, warming that occurs more rapidly, and warming that is sustained over longer time periods.    Reducing the amount of future climate change is called mitigation of climate change. Mitigation is defined as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere. Studies indicate substantial potential for future reductions in emissions by a combination of emission-reducing activities such as energy conservation, increased energy efficiency, and satisfying more of society’s power demands with renewable energy and nuclear energy sources.   Climate mitigation also includes acts to enhance natural sinks, such as reforestation.  Other policy responses include adaptation to climate change. Adaptation to climate change may be planned, either in reaction to or anticipation of climate change, or spontaneous, i.e., without government intervention. Planned adaptation is already occurring on a limited basis.   The barriers, limits, and costs of future adaptation are not fully understood.

Most scientists agree that humans are contributing to observed climate change.   A study of academic papers concerning global warming found that among those whose abstracts expressed a position on the cause of global warming, 97.2% supported the consensus view that it is man-made.  National science academies have called on world leaders for policies to cut global emissions.    In scientific literature, there is a strong consensus that global surface temperatures have increased in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases. No scientific body of national or international standing disagrees with this view. 

The global warming controversy refers to a variety of disputes, substantially more pronounced in the popular media than in the scientific literature, regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature,  especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, whether humankind has contributed significantly to it,  and whether the increase is wholly or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, and what the consequences of global warming will be.

Kathy Kiefer

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