Global Warming
The science of how Earth's climate is changing — its causes, its mechanisms, and its consequences
A. Global warming — the long-term increase in Earth's average surface temperature — is the central feature of the broader phenomenon of climate change that has dominated environmental policy debate for the past three decades. The scientific consensus that human activities are the primary cause of the warming observed since the mid-twentieth century is among the most robust in contemporary science: the Intergovernmental Panel on Climate Change, whose assessment reports synthesise the findings of thousands of published studies, has concluded with high confidence that global average surface temperature has increased by approximately 1.1°C above pre-industrial levels and that virtually all of this warming is attributable to human- caused increases in atmospheric greenhouse gas concentrations.
B. The primary mechanism of human-caused warming is the enhancement of the greenhouse effect. Certain gases in the atmosphere — principally water vapour, carbon dioxide, methane, nitrous oxide, and ozone — absorb and re-emit infrared radiation, trapping heat that would otherwise escape to space and warming the Earth's surface. This natural greenhouse effect is essential for life: without it, Earth's average temperature would be approximately 33°C colder than it currently is. Human activities — principally the combustion of fossil fuels, deforestation, and agricultural practices — have increased the concentrations of carbon dioxide, methane, and nitrous oxide in the atmosphere well above their pre-industrial levels, enhancing the greenhouse effect and causing the surface to warm.
C. The warming of the atmosphere drives a series of feedbacks that amplify or moderate the initial temperature change. The most important amplifying feedback involves water vapour: as the atmosphere warms, it can hold more water vapour, and water vapour is itself a potent greenhouse gas, so warming produces more water vapour which produces more warming. The melting of Arctic sea ice provides another amplifying feedback: ice reflects solar radiation back to space, but the darker ocean surface exposed when ice melts absorbs more radiation, accelerating warming. Moderating feedbacks also exist: increased water vapour promotes cloud formation, and clouds can either warm or cool the Earth depending on their altitude, thickness, and composition.
D. The regional impacts of global warming are highly variable and depend on geography, ocean circulation patterns, and local factors. The Arctic is warming approximately twice as fast as the global average — a phenomenon known as Arctic amplification — with dramatic consequences for sea ice, permafrost, and the livelihoods of communities that depend on traditional ice-based subsistence practices. Sea level is rising as a result of thermal expansion of ocean water and the melting of land-based ice sheets and glaciers, threatening low-lying coastal areas and small island states. Precipitation patterns are shifting, with some regions experiencing more intense rainfall and flooding while others face longer and more severe droughts. Extreme weather events including heatwaves, intense tropical cyclones, and wildfires are becoming more frequent or more intense in many regions.
E. The carbon cycle — the flow of carbon between the atmosphere, oceans, land vegetation, and soils — plays a central role in determining how much warming any given level of emissions produces. The oceans and land vegetation currently absorb approximately half of all human CO₂ emissions, significantly moderating the rate of atmospheric accumulation. However, the capacity of these 'carbon sinks' is not fixed: warmer temperatures and reduced soil moisture may reduce the ability of land ecosystems to absorb carbon, and warming ocean waters absorb CO₂ less efficiently, potentially reducing sink capacity as warming continues. The possibility that carbon sinks could weaken or even turn into carbon sources represents one of the most significant uncertainties in projections of future warming.
F. The international response to global warming has been organised primarily through the United Nations Framework Convention on Climate Change and its successive agreements. The Paris Agreement of 2015 committed signatories to holding global average temperature increase to well below 2°C above pre-industrial levels and pursuing efforts to limit it to 1.5°C, with each country submitting nationally determined contributions specifying its emissions reduction commitments. Scientific assessments indicate that the aggregate of these commitments, as originally submitted, was insufficient to meet the 1.5°C target, and that closing the gap between commitments and the required emissions reductions remains the central challenge of international climate policy.