Climate change

• 1. Introduction

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  In 2024, Earth recorded its warmest year since regular tracking began in 1850, sitting at +1.60 C-change above the pre-industrial baseline. The air now holds roughly 50% more carbon dioxide than it did at the end of the pre-industrial era. That is a level not seen for millions of years. Behind those numbers sits a single driver. Human activity, especially the burning of coal, oil, and natural gas since the Industrial Revolution, has loaded the atmosphere with gases that trap heat. What follows is a story of how scientists first sensed this warming, how it is reshaping oceans and ice and weather, who suffers first and who pays last, and why nearly 200 nations keep meeting to argue over a single number written into the Paris Agreement. The threat is named, the cause is known, and yet the questions about what to do with that knowledge remain open.

• 2. Naming The Problem

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  Before the 1980s, scientists could not yet say whether the warming from greenhouse gases outweighed the cooling from airborne pollution particles. They reached for a cautious phrase, inadvertent climate modification, to describe what humans were doing to the sky. The uncertainty was real. Sulfur dioxide from dirty fuels was reflecting sunlight back to space, masking the heat that carbon dioxide was adding.

  In 1975 the term global warming first appeared, and it became the popular label after NASA climate scientist James Hansen used it in his 1988 testimony before the U.S. Senate. The two terms are not interchangeable to a scientist. Global warming refers only to the rise in global average surface temperature. Climate change covers that warming and its ripple effects, including shifts in where and how much it rains.

  The word anthropogenic gets used when the point is to stress human fault. Since the 2000s, climate change has been the more common phrase, while some scientists, politicians, and outlets prefer climate crisis, climate emergency, or global heating. The choice of word is rarely neutral. It signals how urgent the speaker thinks the matter has become, and that urgency would soon spill from laboratories into national parliaments.

• 3. Reading The Temperature

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  Around 1850, thermometer records began to provide global coverage, and that date became the starting line against which all later warming is measured. Between the 18th century and 1970, there was little net warming, because the heat from greenhouse gases was canceled by cooling from sulfur dioxide emissions that dimmed the planet. After 1970, with pollution controls tightening and greenhouse gases still accumulating, temperature climbed sharply at a rate of around 0.2 C per decade.

  The 2014-2023 decade averaged 1.19 C above the 1850-1900 baseline, within a range of 1.06-1.30 C. Not every year beats the last. Internal variability can push any single year 0.2 C warmer or colder than the trend. From 1998 to 2013, negative phases of the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation slowed warming into what was called the global warming hiatus. This is why temperature change is defined over a 20-year average, which filters out the noise of hot and cold years to reveal the long-term signal.

  The upper atmosphere is cooling even as the surface warms, a fingerprint that points straight at greenhouse gases trapping heat below. Warming forces glaciers to retreat and thins snow cover, while greater ocean evaporation feeds heavier rain. Plants are flowering earlier in spring, and thousands of animal species have permanently moved toward cooler ground, the living edge of a planet finding new boundaries.

• 4. An Uneven Heat

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  Different regions warm at different rates, and the pattern has nothing to do with where the gases were released, because they linger long enough to spread across the whole planet. Since the pre-industrial period, land has warmed almost twice as fast as the global average, since oceans shed heat through evaporation and can store enormous amounts of it. More than 90% of the extra thermal energy in the climate system has gone into the ocean. The rest has heated the atmosphere, melted ice, and warmed the continents.

  Arctic surface temperatures are rising between three and four times faster than the rest of the world. The Northern Hemisphere holds far more land, seasonal snow, and sea ice than the south. When those bright surfaces melt and darken, they flip from reflecting sunlight to absorbing it, and black carbon settling on snow deepens the effect. As ice sheets near the poles melt, they weaken both the Atlantic and Antarctic limbs of thermohaline circulation, the great conveyor that redistributes heat and rain. The energy hidden in that warming ocean is a debt the planet has not yet finished paying out.

• 5. The Greenhouse Engine

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  Greenhouse gases let sunlight pass through to warm the surface, then catch the heat the Earth radiates back, slowing its escape into space. Water vapour, at roughly 50%, and clouds, at roughly 25%, are the biggest contributors to the greenhouse effect. But they shift mainly as a function of temperature, so scientists treat them as feedbacks rather than primary drivers. Gases like carbon dioxide, tropospheric ozone, CFCs, and nitrous oxide change independently of temperature, which makes them the external forcings that move global temperatures.

  Before the Industrial Revolution, natural greenhouse gases kept the surface about 33 C warmer than it would otherwise have been. By 2022, carbon dioxide had risen about 50% and methane about 164% since 1750, with methane levels far above anything in the last 800,000 years. In 2019, human-caused emissions equaled 59 billion tonnes of carbon dioxide equivalent. Of that, 75% was carbon dioxide, 18% methane, 4% nitrous oxide, and 2% fluorinated gases.

  Carbon dioxide comes mainly from burning fossil fuels for transport, manufacturing, heating, and electricity, with more from deforestation and from making cement, steel, aluminium, and fertilizer. Methane rises from livestock, manure, rice fields, landfills, wastewater, and the mining of coal, oil, and gas. The two gases age differently. Methane lasts an average of 12 years, while carbon dioxide can stay for far longer. Land sinks pull down about 29% of annual emissions and the ocean has absorbed 20 to 30% over the last two decades, but carbon is only locked away for good when it settles into the Earth's crust, a process that takes millions of years.

• 6. Oceans, Ice, And Tipping Points

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  Global sea level is rising from thermal expansion and from melting glaciers and ice sheets, reaching 4.8 cm per decade between 2014 and 2023. Over this century, projections range from 32-62 cm under low emissions to 65-101 cm under very high emissions, and instability in Antarctic ice sheets could push that toward a 2-meter rise by 2100. Ice-free Arctic summers, rare at 1.5 C of warming, would arrive once every three to ten years at 2 C. As oceans absorb more carbon dioxide they grow more acidic, and warmer water holds less oxygen, so dead zones are spreading.

  Beyond certain thresholds lie tipping points, lines past which some changes cannot be undone even if temperatures later fall. The Greenland ice sheet is already melting, but at warming between 1.7 C and 2.3 C its loss continues until it vanishes entirely. Ice sheets melt over millennia, yet other thresholds move faster. The collapse of the Atlantic meridional overturning circulation, or irreversible harm to the Amazon rainforest and coral reefs, could unfold in mere decades.

  Wildlife is already on the move. The range of hundreds of North American birds has shifted northward at an average of 1.5 km per year over the past 55 years. In the sea, species have migrated toward the poles faster than those on land, while heatwaves bleach coral reefs and acidification leaves mussels, barnacles, and corals struggling to build their shells. Almost half of global wetlands have disappeared, a loss that hints at how much of the living coastline is now under strain.

• 7. The Human Toll

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  The World Health Organization calls climate change one of the biggest threats to global health in the 21st century. According to the World Economic Forum, 14.5 million more deaths are expected by 2050. Today 30% of the global population lives where extreme heat and humidity are already tied to excess deaths, a share that could reach 50% to 75% by 2100. Drought-driven malnutrition alone could cause 3.2 million deaths by 2050.

  Farming feels the strain in uneven ways. Total crop yields have risen over the past 50 years through better methods, yet climate change has already slowed that growth, helping high-latitude regions while hurting the mid and low latitudes. With 2 C of warming, global livestock numbers could fall 7-10% by 2050 as feed grows scarce. The burden lands hardest on the least responsible. Poorer communities emit little, adapt least, and suffer most.

  The World Bank estimates climate change could push over 120 million people into extreme poverty between 2016 and 2030 without adaptation. At 4 C of warming, labour capacity in South-East Asia and sub-Saharan Africa could drop by 30 to 50% as heat halts outdoor work. Sea level rise threatens low-lying island nations like the Maldives and Tuvalu with statelessness, and in worst-case scenarios models project that areas home to nearly a third of humanity could become Sahara-like and uninhabitable. Some of the most vulnerable cannot even flee, becoming trapped populations without the resources to move.

• 8. The Long Argument

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  Scientists in the 19th century, among them Alexander von Humboldt, began to foresee the effects of a changing climate. In the 1820s, Joseph Fourier proposed the greenhouse effect to explain why Earth was warmer than sunlight alone could make it. In 1856, Eunice Newton Foote showed that air with carbon dioxide warmed more than dry air, writing that an atmosphere of that gas would give the Earth a high temperature. Starting in 1859, John Tyndall established that nitrogen and oxygen are transparent to radiated heat while water vapour, methane, and carbon dioxide absorb and re-radiate it.

  Svante Arrhenius published the first climate model of its kind in 1896, calculating that doubling carbon dioxide would raise temperature by around 5-6 C. Skeptics insisted the effect was saturated and the climate self-regulating. Guy Stewart Callendar, beginning in 1938, gathered evidence that warming and carbon dioxide were both rising, and met the same objections. In the 1950s, Gilbert Plass built a detailed computer model that predicted warming, Roger Revelle showed the oceans would not simply soak up the excess, and Charles Keeling began the steady record now known as the Keeling Curve.

  The National Research Council's 1979 Charney Report backed the models forecasting significant warming. James Hansen's 1988 Senate testimony brought human causation to the public, and the Intergovernmental Panel on Climate Change was set up that same year to advise the world's governments. There is now a nearly unanimous scientific consensus, with agreement in recent literature above 99% as of 2019, and the 2021 IPCC report calling human causation unequivocal. Yet a denial movement, born in the United States and spread to Canada and Australia, has borrowed the tobacco industry's playbook of manufacturing doubt to stall action. The argument over the science was settled long ago; the argument over what to do with it has only grown louder.