Climate change is reshaping our planet. Human activities, especially burning fossil fuels and deforestation, are pumping greenhouse gases into the atmosphere at unprecedented rates. This is trapping heat and disrupting Earth's delicate energy balance.

The evidence is clear: rising temperatures, melting ice, and shifting ecosystems. Scientists use various tools to track these changes, from satellite data to tree rings. Understanding the causes and impacts of climate change is crucial for addressing this global challenge.

Anthropogenic Causes of Climate Change

Fossil Fuel Combustion and Greenhouse Gas Emissions

• Burning of fossil fuels, such as coal (bituminous and anthracite), oil (petroleum), and natural gas (methane), releases carbon dioxide (CO2) and other greenhouse gases into the atmosphere
• CO2 is the primary greenhouse gas emitted through human activities, accounting for about 76% of total anthropogenic greenhouse gas emissions
• Other greenhouse gases released by fossil fuel combustion include methane (CH4), nitrous oxide (N2O), and water vapor
• The transportation sector (cars, trucks, ships, and planes) is a major contributor to fossil fuel-related emissions, along with electricity generation and industrial processes

Land-Use Changes and Deforestation

• Deforestation and land-use changes reduce the ability of Earth's surface to absorb CO2 through photosynthesis and can also release stored carbon into the atmosphere
• Forests act as carbon sinks, storing large amounts of carbon in their biomass and soils; when trees are cut down or burned, this stored carbon is released back into the atmosphere as CO2
• Agriculture and urbanization are major drivers of land-use change, as natural habitats are converted to farmland or developed for human settlements
• Deforestation is particularly prevalent in tropical regions (Amazon rainforest and Southeast Asia), where vast areas of forest are cleared for agriculture, logging, and mining

Agricultural Practices and Methane Emissions

• Agricultural practices, such as livestock farming (cattle and sheep) and rice cultivation, contribute to methane (CH4) emissions
• Methane is a potent greenhouse gas with a global warming potential 28-36 times greater than CO2 over a 100-year period
• Livestock, particularly cattle, produce methane through enteric fermentation during digestion and manure management
• Rice cultivation in flooded paddies creates anaerobic conditions that favor methane-producing bacteria, leading to significant CH4 emissions
• Synthetic fertilizers used in agriculture also contribute to nitrous oxide (N2O) emissions, another powerful greenhouse gas

Industrial Processes and Greenhouse Gas Emissions

• Industrial processes, such as cement production and the use of refrigerants, release CO2 and other greenhouse gases
• Cement production involves the calcination of limestone (CaCO3), which releases CO2 as a byproduct; the cement industry accounts for about 8% of global CO2 emissions
• Hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) are potent greenhouse gases used in refrigeration, air conditioning, and industrial processes; these gases have global warming potentials hundreds to thousands of times greater than CO2
• The production of metals, chemicals, and electronics also contributes to greenhouse gas emissions through energy use and process-related emissions

Greenhouse Gases and Climate

The Greenhouse Effect and Earth's Energy Balance

• Greenhouse gases, such as CO2, CH4, water vapor, and nitrous oxide (N2O), absorb and re-emit infrared radiation, trapping heat in the atmosphere
• The greenhouse effect is a natural process that warms the Earth's surface, making it habitable for life as we know it; without it, the Earth's average surface temperature would be about 33°C colder
• Incoming solar radiation is absorbed by the Earth's surface and atmosphere, and some of this energy is re-emitted as infrared radiation (heat)
• Greenhouse gases absorb this infrared radiation and re-emit it back towards the surface, effectively trapping heat in the lower atmosphere

Anthropogenic Enhancement of the Greenhouse Effect

• Anthropogenic activities have increased the concentration of greenhouse gases in the atmosphere, leading to an enhanced greenhouse effect and global warming
• Since the start of the Industrial Revolution (mid-18th century), atmospheric CO2 concentrations have increased by over 40%, from about 280 ppm to 415 ppm in 2021
• Methane concentrations have more than doubled over the same period, from about 720 ppb to over 1,850 ppb
• This rapid increase in greenhouse gas concentrations is primarily due to human activities, such as fossil fuel combustion, deforestation, and agriculture
• The enhanced greenhouse effect is causing the Earth's surface and lower atmosphere to warm, leading to changes in climate patterns, sea level rise, and other impacts

Radiative Forcing and Feedback Loops

• The radiative forcing of a greenhouse gas depends on its concentration, the wavelengths of radiation it absorbs, and its atmospheric lifetime
• Radiative forcing is a measure of the change in energy flux (W/m2) at the top of the atmosphere due to a change in a climate driver, such as greenhouse gas concentrations
• Positive radiative forcing leads to warming, while negative radiative forcing leads to cooling; the magnitude of the forcing depends on the specific properties of the greenhouse gas
• Feedback loops, such as the melting of Arctic sea ice and the release of methane from thawing permafrost, can amplify the warming effect of greenhouse gases
• As Arctic sea ice melts, it exposes darker ocean waters that absorb more solar radiation, leading to further warming and more ice melt (ice-albedo feedback)
• Thawing permafrost releases stored methane and CO2, contributing to additional warming and further permafrost thaw (permafrost carbon feedback)

Evidence for Climate Change

Temperature Records and Global Warming Trends

• Global temperature records show a clear warming trend since the late 19th century, with the most rapid warming occurring in recent decades
• The Earth's average surface temperature has increased by about 1.0°C since the pre-industrial period (1850-1900), with two-thirds of the warming occurring since 1975
• The past five years (2016-2020) have been the warmest on record, with 2020 tying with 2016 for the warmest year
• Warming is not uniform across the globe; some regions, such as the Arctic, are warming much faster than the global average (Arctic amplification)
• The frequency and intensity of heatwaves and warm temperature extremes have increased, while cold extremes have decreased

Cryosphere Changes and Sea Level Rise

• Satellite and surface-based observations reveal a decline in the extent and thickness of Arctic sea ice, indicating a warming trend in the polar regions
• September Arctic sea ice extent has decreased by about 13% per decade since 1979, with the lowest extents occurring in recent years (2012 and 2020)
• Glaciers and ice sheets worldwide are retreating and losing mass, contributing to sea-level rise
• The Greenland and Antarctic ice sheets have lost an average of 279 and 148 billion tons of ice per year, respectively, between 1993 and 2019
• Mountain glaciers in the Alps, Himalayas, Andes, and other regions are also retreating rapidly, with many expected to disappear by the end of the 21st century
• Global mean sea level has risen by about 21-24 cm since 1880, with the rate of rise accelerating in recent decades due to thermal expansion of the oceans and melting of land-based ice

Ecological Responses and Phenological Changes

• Changes in the timing of seasonal events, such as the arrival of spring and the migration of species, provide evidence of a shifting climate
• Many plant species are flowering earlier in the spring, while some migratory birds are arriving at their breeding grounds earlier and departing later in the fall
• The geographic ranges of many plant and animal species are shifting poleward and to higher elevations as temperatures warm
• Warmer temperatures and changing precipitation patterns are altering the distribution and abundance of species, leading to changes in ecosystem structure and function
• Phenological mismatches, where the timing of interdependent species' life cycles becomes out of sync, can disrupt ecological relationships and food webs

Paleoclimate Records and Historical Context

• Paleoclimate records, such as ice cores, tree rings, and sediment layers, show that current warming is occurring at an unprecedented rate compared to natural climate variations in the past
• Ice cores from Greenland and Antarctica provide a record of atmospheric greenhouse gas concentrations and temperature proxies (δ18O) going back hundreds of thousands of years
• These records show that current CO2 levels (415 ppm) are higher than at any time in at least the past 800,000 years, and possibly the past 3 million years
• Tree rings and other proxy data indicate that the current rate of warming is exceptional in the context of the past 2,000 years, with the last decade being the warmest on record
• Sediment cores from lakes and oceans contain fossils and geochemical indicators that provide evidence of past climate conditions and show that the current rate of change is unprecedented in the geological record

Measuring and Monitoring Climate Change

Surface-Based Observations and Temperature Records

• Surface temperature measurements are collected from weather stations, ships, and buoys worldwide, providing a long-term record of global temperature changes
• The global network of weather stations, coordinated by the World Meteorological Organization (WMO), provides daily measurements of temperature, precipitation, and other variables
• Ship-based measurements of sea surface temperature (SST) have been collected since the mid-19th century, providing a valuable record of ocean temperature changes
• Drifting buoys and moored buoys, such as those deployed by the Global Drifter Program and the Tropical Atmosphere Ocean (TAO) array, provide additional SST measurements and other oceanographic data

Satellite Remote Sensing and Global Observations

• Satellite observations, such as those from NASA's Earth Observing System, provide global coverage of temperature, precipitation, and other climate variables
• Instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Very High Resolution Radiometer (AVHRR) measure land and ocean surface temperatures from space
• Microwave radiometers, such as the Special Sensor Microwave/Imager (SSM/I) and the Advanced Microwave Scanning Radiometer (AMSR-E), provide data on sea ice extent, snow cover, and soil moisture
• Radar altimeters, such as those on the TOPEX/Poseidon and Jason satellite missions, measure sea surface height and ocean circulation patterns
• Gravity Recovery and Climate Experiment (GRACE) satellites measure changes in Earth's gravity field, which can be used to track changes in ice sheet mass and sea level

Paleoclimate Proxies and Historical Climate Data

• Climate proxy data, such as tree rings, ice cores, and coral growth patterns, provide indirect evidence of past climate conditions and help to put current changes into a longer-term context
• Tree rings provide a record of temperature and precipitation variability over the past few thousand years, with the width and density of the rings reflecting growing season conditions
• Ice cores from polar and alpine glaciers contain bubbles of ancient air that provide a direct record of past atmospheric greenhouse gas concentrations, as well as dust and other particles that reflect past climate conditions
• Coral growth patterns, as recorded in the calcium carbonate skeletons of reef-building corals, provide a record of sea surface temperature, salinity, and other ocean conditions over the past several centuries
• Historical records, such as ship logs, agricultural records, and personal diaries, can also provide valuable information about past climate conditions and extreme events

Climate Models and Future Projections

• Climate models, which simulate the Earth's climate system using mathematical equations and physical principles, are used to project future climate changes based on different emission scenarios and to attribute observed changes to specific causes
• General Circulation Models (GCMs) simulate the large-scale circulation of the atmosphere and oceans, as well as the interactions between the land surface, cryosphere, and biosphere
• Earth System Models (ESMs) build on GCMs by incorporating additional components, such as the carbon cycle, dynamic vegetation, and atmospheric chemistry, to provide a more comprehensive representation of the climate system
• Regional Climate Models (RCMs) provide higher-resolution simulations of climate change impacts at the regional and local scales, which are important for adaptation planning and risk assessment
• Climate models are continuously improved and validated against observations, and they are used to project future changes in temperature, precipitation, sea level, and other variables under different greenhouse gas emission scenarios (Representative Concentration Pathways, or RCPs)