Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about the Earth and its climate on a global scale. This body of data, collected over many years, reveals the signals of a changing climate.
Glimpses on the analytical data: As per the latest measurement the Earth’s average surface temperature has risen about 0.9 °C since the late 19th century, a change driven largely by increased carbon-dioxide and other human-made emissions into the atmosphere. Most of the warming occurred in the past 35 years, with the five warmest years on record taking place since 2010. Not only was 2016 the warmest year on record, but eight 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 respective months. The oceans have absorbed much of this increased heat, with the top 700 meters (about 2,300 feet) of ocean showing warming of more than 0.4 °F since 1969. As per the latest measurement of November 2018, the CO2 levels in the atmosphere have increased to 410 ppm. Humans have increased atmospheric CO2 concentration by more than a third since the Industrial Revolution began (18th century). In response to warming, the Greenland and Antarctic ice sheets have decreased in mass. Both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades. Data from NASA’s Gravity Recovery and Climate Experiment show Greenland lost an average of 281 billion tons of ice per year between 1993 and 2016, while Antarctica lost about 119 billion tons during the same time period. Glaciers are retreating almost everywhere around the world — including in the Alps, Himalayas, Andes, Rockies, Alaska and Africa. Satellite observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and that the snow is melting earlier. Global sea level rose about 8 inches in the last century. The rate in the last two decades, however, is nearly double that of the last century and is accelerating slightly every year. Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30%. This increase is the result of humans emitting more CO2 into the atmosphere and hence more being absorbed into the oceans. The amount of CO2 absorbed by the upper layer of the oceans is increasing by about 2 billion tons per year.
Causes of the climate change: An important issue is whether climate variability is due to processes occurring on the Earth, or due to the changes in the intensity of sunlight. However, the geological evidences strongly support earthly and not solar causes. The geological record of ancient climate is excellent and the ancient temperatures can be reconstructed very precisely from the fossil shells of corals and other marine organisms, plants and animals that lived during a given time and indicate whether the climate was wet 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 million years ago, to an unusually cool period, colloquially called the Ice Age, which ended just 7,000 years ago. The earth was so warm (35°F) during the Eocene Optimum that Antarctica was ice-free; ice caps did not start to form there until about 35 million years ago. Palm trees grew at high latitudes and cold-blooded animals, such as crocodiles, lived in the Arctic. Ice Age CO2 levels are well known, because bubbles of Ice Age air are preserved within the Antarctic and Greenland glaciers. These measurements show convincingly that the long-term cooling trend over the last 50 million years is associated with a gradual decrease in CO2 levels, from 2000-3000 ppm during the Eocene Optimum to 200 ppm during the Ice Age. The cause of this decrease is possibly because an increasing amount of carbon was tied up in sedimentary rocks such as limestone.
However, it is reasonable to assume that changes in the sun’s energy (solar irradiance) output would cause the climate to change, since the sun is the fundamental source of energy that drives our climate system. Climate during the Ice Age has been particularly unstable, with many swings of more than 10°F. These fluctuations are recorded in the annual layers of snow preserved in glaciers and in marine sediments. The timing of these swings closely follows regular fluctuations in the 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 climate changes. For example, a decrease in solar activity is thought to have triggered the Little Ice Age between approximately 1650 and 1850, when Greenland was largely 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 be explained by changes in energy from the sun. Because, since 1750, the average amount of energy coming from the sun either remained constant or increased slightly. If the warming were caused by a more active sun, then warmer temperatures would be expected in all layers of the atmosphere. Instead, a cooling in the upper atmosphere with a warming at the surface and in the lower parts of the atmosphere is observed.
Interestingly, most climate scientists agree that the main cause of the current global warming trend is human expansion of the “greenhouse effect”. On Earth, human activities are changing the natural greenhouse. Over the last century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric CO2. To a lesser extent, the clearing of land for agriculture, industry and other human activities has increased concentrations of greenhouse gases. The industrial activities that our modern civilization depends upon have raised atmospheric CO2 levels from 280-400 ppm in the last 150 years. In the Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC), concluded there’s a better than 95% probability that human-produced greenhouse gases such as CO2, CH4 and N2O have caused much of the observed increase in Earth’s temperatures over the past 50 years.
Effects of the climate change: Changing global temperature can cause a rise or fall in sea level due to the accumulation or melting of glacial ice. This effect is global in extent 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 that sea level was higher by about 200 feet at times, such as during the Eocene Optimum, when Antarctica was ice-free and was about 400 feet lower during the height 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 the Industrial Era and then to 400 ppm attributed to the burning of fossil fuels such as coal and petrol etc.
The consequences of changing the natural atmospheric greenhouse are difficult to predict, but certain effects seem likely. For example, on average, Earth will become warmer. Some regions may welcome warmer temperatures, but others may not. Warmer conditions will probably lead to more evaporation and precipitation overall, but individual regions will vary, some becoming wetter and others dryer. A stronger greenhouse effect will warm the oceans and partially melt glaciers and other ice, increasing sea level. Ocean water also will expand if it warms, contributing further to sea level rise. Meanwhile, some crops and other plants may respond favorably to increased atmospheric CO2, growing more vigorously and using water more efficiently. At the same time, higher temperatures and shifting climate patterns may change the areas where crops grow best and affect the makeup of natural plant communities. The potential future effects of global climate change include more frequent wildfires, longer periods of drought in some regions and an increase in the number, duration and intensity of tropical storms.
Scientists have high confidence that global temperatures will continue to rise for decades to come, largely due to the input of large quantities of greenhouse gases produced by human activities. According to the IPCC, the extent of climate change effects on individual regions will vary over time and with the ability of different societal and environmental systems to mitigate or adapt to change. The IPCC predicts that increases in global mean temperature of less than 1-3°C above 1990 levels will produce beneficial impacts in some regions and harmful ones in others.
Dr. Riyaz Ahmad Mir [Sr. Geologist] is working with Geological Survey of India, State Unit: J&K, Rajbagh Srinagar.