What We Know About Climate Change So Far

Md. Arafat Rahman

When it was unclear whether warming by greenhouse gases would dominate aerosol-induced cooling, scientists often used the term inadvertent climate modification to refer to humankind’s impact on the climate. In the 1980s, the terms global warming and climate change were popularised, the former referring only to increased surface warming, while the latter describes the full effect of greenhouse gases on the climate. In the 2000s, the term climate change increased in popularity. Global warming usually refers to human-induced warming of the Earth system, whereas climate change can refer to natural as well as anthropogenic change. The two terms are often used interchangeably. 

The largest driver of warming is the emission of gases that create a greenhouse effect, of which more than 90% are carbon dioxide (CO2) and methane. Fossil fuel burning for energy consumption is the main source of these emissions, with additional contributions from agriculture, deforestation, and manufacturing. The human cause of climate change is not disputed by any scientific body of national or international standing. Temperature rise is accelerated or tempered by climate feedbacks, such as loss of sunlight-reflecting snow and ice cover, increased water vapour, and changes to land and ocean carbon sinks.

Temperature rise on land is about twice the global average increase, leading to desert expansion and more common heat waves and wildfires. Temperature rise is also amplified in the Arctic, where it has contributed to melting permafrost, glacial retreat and sea ice loss. Warmer temperatures are increasing rates of evaporation, causing more intense storms and weather extremes. Impacts on ecosystems include the relocation or extinction of many species as their environment changes, almost immediately in coral reefs, mountains, and the Arctic. 

Climate change threatens people with food insecurity, water scarcity, flooding, infectious diseases, extreme heat, economic losses, and displacement. These impacts have led the World Health Organization to call climate change the greatest threat to global health in the 21st century. Even if efforts to minimize future warming are successful, some effects will continue for centuries, including rising sea levels, rising ocean temperatures, and ocean acidification. Under the 2015 Paris Agreement, nations collectively agreed to keep warming under 2.0 °C through mitigation efforts. However, with pledges made under the Agreement, global warming would still reach about 2.8 °C by the end of the century. Limiting warming to 1.5 °C would require halving emissions by 2030 and achieving near-zero emissions by 2050. 

Responding to climate change involves mitigation and adaptation. Mitigation consists of reducing greenhouse gas emissions and removing them from the atmosphere; methods include the development of low-carbon energy sources such as wind and solar, a phase-out of coal, enhanced energy efficiency, reforestation, and forest preservation. Adaptation consists of adjusting to the actual or expected climate, such as through improved coastline protection, better disaster management, assisted colonization, and the development of more resistant crops. Adaptation alone cannot avert the risk of severe, widespread and irreversible impacts. 

The Northern Hemisphere and the North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more seasonal snow cover and sea ice, because of how the landmasses are arranged around the Arctic Ocean. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat. Localised black carbon deposits on snow and ice also contribute to Arctic warming. Arctic temperatures have increased and are predicted to continue to increase during this century at over twice the rate of the rest of the world. The melting of glaciers and ice sheets in the Arctic disrupts ocean circulation, including a weakened Gulf Stream, further changing the climate. 

Evidence of warming from air temperature measurements are reinforced with a wide range of other observations. There has been an increase in the frequency and intensity of heavy precipitation, melting of snow and land ice, and increased atmospheric humidity. Flora and fauna are also behaving in a manner consistent with warming; for instance, plants are flowering earlier in spring. Another key indicator is the cooling of the upper atmosphere, which demonstrates that greenhouse gases are trapping heat near the Earth’s surface and preventing it from radiating into space. 

While locations of warming vary, the patterns are independent of where greenhouse gases are emitted, because the gases persist long enough to diffuse across the planet. Since the pre-industrial period, global average land temperatures have increased almost twice as fast as global average surface temperatures. This is because of the larger heat capacity of oceans, and because oceans lose more heat by evaporation. Over 90% of the additional energy in the climate system over the last 50 years has been stored in the ocean, with the remainder warming the atmosphere, melting ice, and warming the continents. 

The climate system is experiencing various cycles on its own which can last for years, decades or even centuries. Other changes are caused by an imbalance of energy that is external to the climate system, but not always external to the Earth. Examples of external forcings include changes in the composition of the atmosphere, solar luminosity, volcanic eruptions, and variations in the Earth’s orbit around the Sun. To determine the human contribution to climate change, known internal climate variability and natural external forcing need to be ruled out. Attribution to recent climate change shows that the primary driver is elevated greenhouse gases, but that aerosols also have a strong effect. 

Human activity since the Industrial Revolution, mainly extracting and burning coal, oil, and natural gas has increased the number of greenhouse gases in the atmosphere, resulting in a radioactive imbalance. In 2018, the concentrations of CO2 and methane had increased by about 45% and 160%, respectively, since 1750. These CO2 levels are much higher than they have been at any time during the last 800,000 years, the period for which reliable data have been collected from air trapped in ice cores. Less direct geological evidence indicates that CO2 values have not been this high for millions of years. 

Despite the contribution of deforestation to greenhouse gas emissions, the Earth’s land surface, particularly its forests, remain a significant carbon sink for CO2. Natural processes, such as carbon fixation in the soil and photosynthesis, more than offset the greenhouse gas contributions from deforestation. The land surface sink is estimated to remove about 29% of annual global CO2 emissions. The ocean also serves as a significant carbon sink via a two-step process. First, CO2 dissolves in the surface water. Afterwards, the ocean’s overturning circulation distributes it deep into the ocean’s interior, where it accumulates over time as part of the carbon cycle. Over the last two decades, the world’s oceans have absorbed 20 to 30% of emitted CO2. 

Air pollution, in the form of aerosols, not only puts a large burden on human health, but also affects the climate on a large scale. A gradual reduction in the amount of sunlight reaching the Earth’s surface was observed, a phenomenon is popularly known as global dimming, typically attributed to aerosols from biofuel and fossil fuel burning. Aerosol removal by precipitation gives tropospheric aerosols an atmospheric lifetime of only about a week, while stratospheric aerosols can remain in the atmosphere for a few years. Globally, aerosols have been declining since 1990, meaning that they no longer mask greenhouse gas warming as much. 

Humans change the Earth’s surface mainly to create more agricultural land. Today, agriculture takes up 34% of Earth’s land area, while 26% is forest, and 30% is the glacier, desert, etc. The amount of forested land continues to decrease, largely due to conversion to cropland in the tropics. This deforestation is the most significant aspect of land surface change affecting global warming. The main causes of deforestation are the permanent land-use change from forest to agricultural land producing products such as beef and palm oil, logging to produce forest products, short term shifting cultivation, and wildfires. 

In addition to affecting greenhouse gas concentrations, land-use changes affect global warming through a variety of other chemical and physical mechanisms. Changing the type of vegetation in a region affects the local temperature, by changing how much of the sunlight gets reflected into space, and how much heat is lost by evaporation. Deforestation can also contribute to changing temperatures by affecting the release of aerosols and other chemical compounds that influence clouds, and by changing wind patterns. 

Explosive volcanic eruptions represent the largest natural forcing over the industrial era. When the eruption is sufficiently strong sunlight can be partially blocked for a couple of years, with a temperature signal lasting about twice as long. In the industrial era, volcanic activity has had negligible impacts on global temperature trends. Present-day volcanic CO2 emissions are equivalent to less than 1% of current anthropogenic CO2 emissions. 

The environmental effects of climate change are broad and far-reaching, affecting oceans, ice, and weather. Changes may occur gradually or rapidly. Evidence for these effects comes from studying climate change in the past, from modelling, and modern observations. Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency. Extremely wet or dry events within the monsoon period have increased. The maximum rainfall and wind speed from hurricanes and typhoons is likely increasing. Climate change has led to decades of shrinking and thinning of the Arctic sea ice, making it vulnerable to atmospheric anomalies.

The long-term effects of climate change include further ice melt, ocean warming, sea-level rise, and ocean acidification. On the timescale of centuries to millennia, the magnitude of climate change will be determined primarily by anthropogenic CO2 emissions. This is due to CO2’s long atmospheric lifetime. Oceanic CO2 uptake is slow enough that ocean acidification will continue for hundreds to thousands of years. These emissions are estimated to have prolonged the current interglacial period by at least 100,000 years. Sea level rise will continue over many centuries, with an estimated rise of 2.3 metres per degree Celsius after 2000 years. 

The oceans have heated more slowly than the land, but plants and animals in the ocean have migrated towards the colder poles faster than species on land. Just as on land, heatwaves in the ocean occur more frequently due to climate change, with harmful effects found on a wide range of organisms such as corals, kelp, and seabirds. Ocean acidification is impacting organisms that produce shells and skeletons, such as mussels and barnacles, and coral reefs. Coral reefs have seen extensive bleaching after heat waves. Harmful algae bloom enhanced by climate change and eutrophication cause anoxia, disruption of food webs and massive large-scale mortality of marine life. Coastal ecosystems are under particular stress, with almost half of wetlands have disappeared as a consequence of climate change and other human impacts. 

The effects of climate change on humans, mostly due to warming and shifts in precipitation, have been detected worldwide. Regional impacts of climate change are now observable on all continents and across ocean regions, with low-latitude, less developed areas facing the greatest risk. Continued emission of greenhouse gases will lead to further warming and long-lasting changes in the climate system for both people and ecosystems. Climate change risks are unevenly distributed but are generally greater for disadvantaged people in developing and developed countries. 

Author is a Columnist & Asst. Officer   

Career & Professional Development Services Department; Southeast University 

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