The global thermostat is set by the amount of solar energy retained by the Earth’s atmosphere. Land and water absorb incoming sunlight and transform it into heat, which is released back into the air as infrared radiation. Like the glass walls of a greenhouse, atmospheric gases, principally CO2, water vapour, and methane trap most of the ascending heat and keep it in the lower atmosphere making Earth hospitable to life.This is commonly known as the greenhouse effect. If the Earth, like the moon, had no greenhouse gases (GHGs), then it would be a much colder place and would not support life. Without greenhouse effect, the average Earth temperature would be around 6.80F (minus 140 C), well below the freezing point of water. But the system’s stability has now been jolted.
The burning of carbon laden fossil fuels had hiked the atmospheric CO2 to levels unprecedented during human history. The Greenhouse Earth is getting warmer. Apart from CO2, there are many other anthropogenic GHGs. They differ from CO2 in their ‘radiative forcing’ (heat trapping capacity) and in their residence time. The relevant and important characteristics of six common GHGs are given in the accompanying table. (see table) The total warming effect of all of the anthropogenic GHGs is determined by adding up the separate ‘radiative forcing’ of each of the six GHGs. In the table, the ‘radiative forcing’ of each GHG has been expressed in unit of CO2 equivalent (CO2 E). For example, since CH4 has a radiative forcing equal to 23 times that of CO2, we may say that each single molecule of CH4 in the atmosphere should be counted as equivalent in warming potential to 23 molecules of CO2. Similarly, each molecule of N2O counts as an equivalent to 296 molecules of CO2. Sea levels remained unchanged for nearly 2,000 years. By releasing CO2 and other heat trapping gases into the atmosphere we have warmed the Earth and raised the sea level.
Tide gauge measurement shows that the current global sea level rise began at the start of the 20th century. Between 1901 and 2018, the globally averaged sea level rose by 15-25 centimetres (6-10 inch). More precise data gathered from satellite radar measurements reveal an accelerating rise of 7.5 cm (3 inches) from 1993 to 2017, for an average rate of 31 mm (1/4 inch) per decade. Based on this finding, the total sea level rise by 2100 (relative to 2000) is projected to reach 70-100 centimetres, depending on which GHG emission pathway we follow. It appears from the table that main GHGs stay in the atmosphere as long as200 years. So even if further emissions were stopped from today, the Earth would keep warming for centuries ~ though probably more slowly.
On a geological time scale, the sea level appears to be linked to average global temperature. In the last ice age when the average temperature was 50C cooler, sea level was approximately 100 meters lower than it is today. In the last interglacial period when temperatures were 1 -2oC higher than today, sea level was approximately 100 meters higher (National Academy 1992:583). Global warming is projected to increase sea levels because of the thermal expansion of water of the oceans and melting ice on land.Between 1993 and 2018, the thermal expansion of water contributed 42 percent to sea level rise; melting of temperate glaciers, 21 percent; Greenland 15 per cent; and Antarctica, 8 percent. Climate scientists expect the rate to further accelerate during the 21st century, with the latest measurements saying the sea levels are rising by 3.7 mm per year. About half of the measured global sea level rise on Earth is from warming waters and thermal expansion.
Usually, as materials are heated, molecules tend to expand and get less dense. If they are cooled,molecules tend to contract and get denser. But an important exception is water. The range that is different for water is between 0o C and 4oC. Water at any other temperature would expand when heated, and contract when cooled. At 4oC, water is at its most contracted state, but it is not frozen. It does not freeze until 0oC. This means as it goes towards 0oC it expands because of the crystallisation of water into ice. This results from a rearrangement of oxygen and hydrogen bonds present in water into stronger and more complex attachments. Because water at 0oC is more expanded than at 4oC, water heated from 0oCwill shrink from its crystalized state until it makes it back to 4oC. Beyond 4 degrees the water will start to expand again.
Ice plays a crucial role in shaping our planet’s environment. Ice ~ in the form of sea ice, glaciers, ice caps and snow – helps to keep the planet cool by reflecting some of the sun’s heat. In contrast,the dark surfaces of the open sea and snow-free ground absorb heat. As ice disappears, the earth retains more of the sun heat. And as the earth warms up, more and more ice melts. Through this feedback process, declining ice strengthens global warming. The earth’s climate is changing. At the moment, this change is most noticeable in the Arctic, where the average temperature has risen at twice the rate as in the rest of the world during the past few decades.
Because the Arctic has fallen victim to a vicious cycle in which sea ice that would normally reflect sunlight melts, and leads to the sun’s heat being absorbed by seawater, raising its temperature and melting ice further. This change has been observed by the Japan Aerospace Exploration Agency’s satellite. Steady observation results obtained by Japan over the past three decades have contributed to assessing the change accurately and forecasting future developments. The continued melting of sea ice in the Arctic Circle is devastating the flora and fauna that are specially adopted to the extremes of the Arctic environment, such as polar bears and ice-dependent seals as well as the people for whom these animals are a primary food source. In addition, the Arctic is the final dumping ground for contaminants, brought by winds and sea currents from the industrial centres of the world. The situation of indigenous people of the Arctic has become precarious. Climate change in the Arctic affects lives all over the world.