Earth-orbiting satellites and other technological advanceshave enabled scientists to see the big picture, collecting many different typesof information about the Earth and its climate on a global scale. This body ofdata, collected over many years, reveals the signals of a changing climate.
Glimpses on the analytical data: As per the latestmeasurement the Earth’s average surface temperature has risen about 0.9 °Csince the late 19th century, a change driven largely by increasedcarbon-dioxide and other human-made emissions into the atmosphere. Most of thewarming occurred in the past 35 years, with the five warmest years on recordtaking place since 2010. Not only was 2016 the warmest year on record, buteight of the 12 months that make up the year — from January through September,with the exception of June — were the warmest on record for those respectivemonths. The oceans have absorbed much ofthis increased heat, with the top 700 meters (about 2,300 feet) of oceanshowing warming of more than 0.4 °F since 1969. As per the latest measurementof November 2018, the CO2 levels in the atmosphere have increased to 410 ppm.Humans have increased atmospheric CO2 concentration by more than a third sincethe Industrial Revolution began (18th century). In response to warming, theGreenland and Antarctic ice sheets have decreased in mass. Both the extent andthickness of Arctic sea ice has declined rapidly over the last several decades.Data from NASA’s Gravity Recovery and Climate Experiment show Greenland lost anaverage of 281 billion tons of ice per year between 1993 and 2016, whileAntarctica lost about 119 billion tons during the same time period. Glaciersare retreating almost everywhere around the world — including in the Alps,Himalayas, Andes, Rockies, Alaska and Africa. Satellite observations revealthat the amount of spring snow cover in the Northern Hemisphere has decreasedover the past five decades and that the snow is melting earlier. Global sealevel rose about 8 inches in the last century. The rate in the last twodecades, however, is nearly double that of the last century and is acceleratingslightly every year. Since the beginning of the Industrial Revolution, theacidity of surface ocean waters has increased by about 30%. This increase isthe result of humans emitting more CO2 into the atmosphere and hence more beingabsorbed into the oceans. The amount of CO2 absorbed by the upper layer of theoceans is increasing by about 2 billion tons per year.
Causes of the climate change: An important issue is whetherclimate variability is due to processes occurring on the Earth, or due to thechanges in the intensity of sunlight. However, the geological evidencesstrongly support earthly and not solar causes. The geological record of ancientclimate is excellent and the ancient temperatures can be reconstructed veryprecisely from the fossil shells of corals and other marine organisms, plantsand animals that lived during a given time and indicate whether the climate waswet or dry. It is reported that the overall climatic trend has been cooling,from an unusually warm period, called the Eocene Optimum, some 55-45 millionyears ago, to an unusually cool period, colloquially called the Ice Age, which endedjust 7,000 years ago. The earth was so warm (35°F) during the Eocene Optimumthat Antarctica was ice-free; ice caps did not start to form there until about35 million years ago. Palm trees grew at high latitudes and cold-bloodedanimals, such as crocodiles, lived in the Arctic. Ice Age CO2 levels are wellknown, because bubbles of Ice Age air are preserved within the Antarctic andGreenland glaciers. These measurements show convincingly that the long-termcooling trend over the last 50 million years is associated with a gradualdecrease in CO2 levels, from 2000-3000 ppm during the Eocene Optimum to 200 ppmduring the Ice Age. The cause of this decrease is possibly because anincreasing amount of carbon was tied up in sedimentary rocks such as limestone.
However, it is reasonable to assume that changes in thesun’s energy (solar irradiance) output would cause the climate to change, sincethe sun is the fundamental source of energy that drives our climate system.Climate during the Ice Age has been particularly unstable, with many swings ofmore than 10°F. These fluctuations arerecorded in the annual layers of snow preserved in glaciers and in marinesediments. The timing of these swings closely follows regular fluctuations inthe tilt of the earth’s axis and the shape of its orbit around the sun.Further, studies show that solar variability has played a role in past climatechanges. For example, a decrease in solar activity is thought to have triggeredthe Little Ice Age between approximately 1650 and 1850, when Greenland waslargely cut off by ice from 1410 to 1720 and glaciers advanced in the Alps.But, there are evidences that show that current global warming cannot beexplained by changes in energy from the sun. Because, since 1750, the averageamount of energy coming from the sun either remained constant or increasedslightly. If the warming were caused by a more active sun, then warmertemperatures would be expected in all layers of the atmosphere. Instead, acooling in the upper atmosphere with a warming at the surface and in the lowerparts of the atmosphere is observed.
Interestingly, most climate scientists agree that the maincause of the current global warming trend is human expansion of the”greenhouse effect”. On Earth, human activities are changing thenatural greenhouse. Over the last century the burning of fossil fuels like coaland oil has increased the concentration of atmospheric CO2. To a lesser extent,the clearing of land for agriculture, industry and other human activities hasincreased concentrations of greenhouse gases. The industrial activities thatour modern civilization depends upon have raised atmospheric CO2 levels from280-400 ppm in the last 150 years. In the Fifth Assessment Report, theIntergovernmental Panel on Climate Change (IPCC), concluded there’s a betterthan 95% probability that human-produced greenhouse gases such as CO2, CH4 andN2O have caused much of the observed increase in Earth’s temperatures over thepast 50 years.
Effects of the climate change: Changing global temperaturecan cause a rise or fall in sea level due to the accumulation or melting ofglacial ice. This effect is global inextent and one that can have an extremely deleterious effect on human beings,since so many live near the coast. The geological evidence is very strong thatsea level was higher by about 200 feet at times, such as during the EoceneOptimum, when Antarctica was ice-free and was about 400 feet lower during theheight of the Ice Age. CO2 levels have risen since the end of the Ice Age,first to a natural level of about 280 ppm just before the start of theIndustrial Era and then to 400 ppm attributed to the burning of fossil fuelssuch as coal and petrol etc.
The consequences of changing the natural atmosphericgreenhouse are difficult to predict, but certain effects seem likely. Forexample, on average, Earth will become warmer. Some regions may welcome warmertemperatures, but others may not. Warmer conditions will probably lead to moreevaporation and precipitation overall, but individual regions will vary, somebecoming wetter and others dryer. A stronger greenhouse effect will warm theoceans and partially melt glaciers and other ice, increasing sea level. Oceanwater also will expand if it warms, contributing further to sea level rise.Meanwhile, some crops and other plants may respond favorably to increasedatmospheric CO2, growing more vigorously and using water more efficiently. Atthe same time, higher temperatures and shifting climate patterns may change theareas where crops grow best and affect the makeup of natural plant communities.The potential future effects of global climate change include more frequentwildfires, longer periods of drought in some regions and an increase in thenumber, duration and intensity of tropical storms.
Scientists have high confidence that global temperatureswill continue to rise for decades to come, largely due to the input of largequantities of greenhouse gases produced by human activities. According to theIPCC, the extent of climate change effects on individual regions will vary overtime and with the ability of different societal and environmental systems tomitigate or adapt to change. The IPCC predicts that increases in global meantemperature of less than 1-3°C above 1990 levels will produce beneficialimpacts in some regions and harmful ones in others.
Dr. Riyaz Ahmad Mir [Sr. Geologist] is working withGeological Survey of India, State Unit: J&K, Rajbagh Srinagar.