The causes and impacts of global warming

The natural greenhouse effect is the process by which naturally occurring greenhouse gases (such as water vapour, carbon dioxide and methane) absorb and re-radiate outgoing longwave radiation, keeping heat in the lower atmosphere.

It is important because it keeps the Earth on average about 33°C warmer than it would otherwise be (around +15°C rather than −18°C), making the planet warm enough to support life.

• Carbon dioxide (CO2) — from the burning of fossil fuels (coal, oil and gas) for electricity, industry and transport, and from deforestation.
• Methane (CH4) — from agriculture, especially cattle (enteric fermentation) and flooded rice paddies, and from landfill waste. Methane is more potent per molecule than CO2.
• Nitrous oxide (N2O) — from the use of nitrogen fertilisers in intensive farming.

(CFCs/halocarbons from refrigerants and aerosols would also be creditworthy.)

A positive feedback mechanism is a process in which an initial change (here, warming) triggers effects that reinforce and amplify the original change, rather than dampening it — the system moves further in the same direction.

Example — permafrost–methane feedback: warming thaws permafrost in the Arctic, which releases stored methane (a greenhouse gas); the extra methane strengthens the greenhouse effect and causes more warming, which thaws yet more permafrost.

(The ice–albedo feedback — melting ice lowering albedo so more heat is absorbed — is an equally valid example.)

Overall direction and magnitude. Atmospheric CO2 has risen substantially. From the pre-industrial ~280 ppm it has increased to over 420 ppm today — a rise of about 140 ppm, or roughly 50%. Since the modern record began at ~315 ppm in 1958, CO2 has risen by about 105 ppm in under 70 years.

Rate. The increase has accelerated: annual rises are larger now (roughly 2–3 ppm per year) than in the early record, so the curve steepens over time — it is not linear.

Anomaly / superimposed pattern. A regular annual sawtooth fluctuation sits on top of the trend: CO2 dips in the northern-hemisphere summer (vegetation photosynthesising) and rises in winter, but each year's minimum is higher than the last, confirming the long-term upward trend.

Overall, the data show a large, accelerating and continuing rise in CO2 well above the natural pre-industrial baseline.

Methane (CH4) is a concern for several linked reasons.

It is a potent greenhouse gas. Molecule for molecule, methane traps far more outgoing longwave radiation than CO2 (it has a much higher global-warming potential over a 100-year period). So even relatively small increases in concentration have a disproportionate warming effect.

Human sources are large and growing. Methane comes from agriculture — cattle and other livestock (enteric fermentation) and flooded rice paddies — from landfill decomposition, and from leaks during fossil-fuel extraction. As global population, meat consumption and waste rise, these sources grow, so atmospheric methane has more than doubled from a pre-industrial level of about 700 ppb to around 1900 ppb.

It drives a dangerous positive feedback. Much methane is stored frozen in permafrost and in ocean-floor hydrates. As warming thaws Arctic permafrost, this stored methane is released, adding to the greenhouse effect and causing further warming, which thaws more permafrost — a self-reinforcing positive feedback that could accelerate warming and is hard to reverse.

However, methane's atmospheric lifetime is shorter than CO2's (it breaks down in roughly a decade), so cutting methane emissions can slow warming relatively quickly — which is why it is targeted in climate policy.

Overall, methane matters because it is highly potent, its human sources are large and rising, and it can trigger a runaway permafrost feedback — making it a key focus for mitigation.

Global warming threatens Bangladesh through a combination of physical and human impacts because it is a low, flat delta with tens of millions of people living close to sea level.

Sea-level rise and permanent inundation. As oceans warm (thermal expansion) and land ice melts, rising seas will permanently flood low-lying coastal land. Because so much of Bangladesh is only a few metres above sea level, even a modest rise could displace millions of people, creating environmental refugees and huge pressure on cities such as Dhaka.

Storm surges and flooding. Warmer seas can make tropical cyclones more intense; combined with higher base sea levels, storm surges push further inland, causing loss of life, destruction of homes and damage to infrastructure. Bangladesh already experiences devastating cyclone flooding, which is projected to worsen.

Saltwater intrusion and agriculture. Rising seas push salt water into coastal soils and groundwater (salinisation). This reduces the fertility of farmland and contaminates drinking water, threatening the rice farming and fishing on which many livelihoods and the national food supply depend.

Water and health impacts. Contaminated water and flooding increase waterborne diseases (e.g. cholera), and the loss of farmland can lead to malnutrition and rural poverty, driving migration to overcrowded cities.

Limited capacity to adapt. As a low-income country, Bangladesh cannot easily afford large-scale sea defences of the kind that protect the wealthy Netherlands, so it relies on cheaper measures (raised shelters, embankments, early-warning systems) that offer only partial protection.

In short, global warming threatens Bangladesh with land loss, more intense flooding, salinised farmland and public-health crises, made worse by its limited financial capacity to defend itself.

Global temperatures have risen by roughly 1.1–1.2°C since the late nineteenth century, and most climate scientists (and the IPCC) conclude that this recent warming is largely human-caused. This answer examines the evidence for that conclusion and weighs it against natural explanations before reaching a judgement.

Evidence pointing to a human cause.

The timing and rate match industrialisation. The sharp acceleration in warming coincides with the industrial era and the huge rise in fossil-fuel use. The Keeling curve shows atmospheric CO2 climbing from a pre-industrial ~280 ppm to over 420 ppm today — a level and rate of change far outside the natural range seen in ice-core records covering hundreds of thousands of years.

The chemical 'fingerprint'. The extra CO2 carries an isotopic signature characteristic of fossil-fuel carbon (depleted in carbon-14 and carbon-13), showing the additional CO2 comes from burning ancient organic material rather than from volcanoes or the oceans. At the same time atmospheric oxygen is falling in step with combustion — consistent with burning, not with natural release.

Pattern of warming. Models and observations show the lower atmosphere warming while the stratosphere cools — the signature expected from an enhanced greenhouse effect, not from increased solar output (which would warm all layers). Nights and winters are warming faster than days and summers, again the greenhouse fingerprint.

Consistency with the mechanism. The physics of the enhanced greenhouse effect is well understood: more greenhouse gases trap more outgoing longwave radiation. The observed warming is quantitatively consistent with the measured rise in these gases plus positive feedbacks (ice-albedo, permafrost methane).

Evaluating natural explanations.

Natural factors do influence climate — Milankovitch orbital cycles, solar variation and volcanic activity have driven past change. However, orbital cycles operate over tens of thousands of years and cannot explain a rise over mere decades; solar output has been roughly flat or slightly declining in recent decades while temperatures climbed; and large volcanic eruptions cause short-term cooling, not sustained warming. Natural variability (e.g. El Niño) adds year-to-year 'noise' but cannot account for the long-term trend.

Uncertainty and counter-arguments. Sceptics note that the climate system is complex and that projections carry uncertainty (feedback strengths, model limits). This is fair — but uncertainty about how much future warming will occur is different from doubt about the cause of the warming already observed. The convergence of independent lines of evidence (rising CO2, its fossil fingerprint, the vertical pattern of warming, and the failure of natural factors to explain it) is what makes the human-cause conclusion so robust.

Conclusion. The weight of evidence strongly supports the view that recent global warming is largely human-caused. Natural factors alone cannot explain warming of this rate and pattern, whereas the enhanced greenhouse effect — driven by fossil-fuel CO2 and other emissions — fits every strand of evidence. Natural variability modulates the trend but does not create it, so on balance the human-cause explanation is the most convincing.

Feedback mechanisms are processes by which an initial warming triggers changes that either amplify it (positive feedback) or dampen it (negative feedback). Because these feedbacks are powerful yet uncertain, they are central to how fast — and how far — future warming will go. This answer examines the main feedbacks and judges their overall effect.

Positive feedbacks that accelerate warming.

Ice–albedo feedback. Warming melts bright, high-albedo ice and snow, exposing darker ocean and land with low albedo that absorb more solar radiation; this warms the surface further and melts still more ice. It is a key reason the Arctic is warming roughly twice as fast as the global average, and it operates quickly.

Permafrost–methane feedback. Warming thaws Arctic permafrost, releasing stored methane and CO2. These strengthen the greenhouse effect, causing more warming and more thaw. Because the carbon stored in permafrost is vast, this is one of the most feared feedbacks and a potential tipping point.

Ocean carbon feedback. Warmer oceans dissolve less CO2 (and warming can weaken the biological pump), so a warming ocean absorbs a smaller share of emissions, leaving more CO2 in the atmosphere and amplifying warming.

Water-vapour feedback. A warmer atmosphere holds more water vapour, itself a greenhouse gas, which enhances warming further.

Negative feedbacks that could slow warming.

The clearest example is cloud feedback: more evaporation can produce more low cloud, which reflects incoming solar radiation and cools the surface, partially offsetting warming. Enhanced plant growth ('CO2 fertilisation') and faster chemical weathering of rock (removing CO2 over long timescales) are further dampening processes. However, cloud feedback is highly uncertain — high clouds can actually trap heat, so the net cloud effect may even be positive.

Weighing the two. The scientific consensus is that positive feedbacks dominate, which is why the climate is sensitive to relatively modest changes in greenhouse gases: the initial warming from human emissions is amplified by ice-albedo, permafrost and water-vapour feedbacks far more than it is offset by cloud feedback. This is precisely why IPCC projections carry a wide range (about 1.5–4.4°C by 2100) — the uncertainty in feedback strength, more than anything except human emission choices, sets the upper end of possible warming. If strong feedbacks and tipping points are triggered, warming could be self-sustaining even if emissions were cut.

Conclusion. Feedback mechanisms are decisive for the rate of future warming: negative feedbacks provide only limited braking, while a suite of powerful positive feedbacks threatens to accelerate warming and push the system past tipping points. On balance, feedbacks make the climate system more dangerous and less predictable than the direct effect of emissions alone would suggest — which is why understanding them is so important for projections and for the urgency of mitigation.

Global warming is a global process, yet its impacts are highly uneven — geographically, economically and socially. This answer assesses that unevenness across physical exposure and human capacity to cope, before judging how far impacts really are unequal.

Physically, some places are far more exposed than others.

Low-lying coasts and small islands face the greatest physical threat from sea-level rise (thermal expansion plus melting land ice). Atoll states such as Tuvalu (~2 m) and the Maldives (averaging ~1.5 m above sea level) risk becoming uninhabitable, and delta nations such as Bangladesh face permanent inundation and worsening storm surges. By contrast, high-altitude or continental interiors are less exposed to the sea, though they face their own hazards.

The Arctic illustrates uneven warming itself: because of the ice-albedo feedback it is warming roughly twice as fast as the global average, transforming ecosystems and indigenous livelihoods far more than the tropics. Climate belts are shifting, so some regions (e.g. parts of the Sahel and the Mediterranean) face worsening drought and desertification, while a few high-latitude areas may see longer growing seasons — a rare example of a potential local 'winner'.

Economically and socially, vulnerability is deeply unequal.

Even where physical hazards are similar, the capacity to adapt differs enormously. The Netherlands has about a third of its land below sea level yet is well protected by the engineered Delta Works dykes and storm barriers — a wealthy country can 'buy' resilience. A low-income country facing comparable exposure cannot afford such defences and relies on cheaper measures (embankments, cyclone shelters, early-warning systems). So the same sea-level rise produces very different outcomes depending on wealth and governance.

LICs are also more sensitive: they depend heavily on climate-vulnerable subsistence agriculture and fishing, have weaker health systems, and less capacity to absorb shocks — so droughts, floods and crop failures translate into more deaths, displacement and lasting poverty. This is compounded by a stark injustice: the countries that have emitted least (small islands, sub-Saharan Africa) face the greatest impacts, while high-emitting industrialised nations are often better placed to protect themselves.

Counter-argument — impacts are also shared and interconnected.

The unevenness can be overstated. Some impacts are global: rising food prices, disrupted supply chains, mass climate migration and pressure on borders affect rich countries too. Wealthy nations are not immune — heatwaves, wildfires and flooding increasingly strike Europe, North America and Australia. And feedbacks/tipping points would raise risks everywhere. So while the severity and ability to cope are uneven, no country fully escapes.

Conclusion. The impacts of global warming will be felt very unevenly, but the unevenness is driven less by physical geography alone than by inequality in wealth and adaptive capacity. Physically, low-lying LICs and small island states and the fast-warming Arctic are most exposed; socially, poverty magnifies these threats while rich nations can defend themselves — as the contrast between Bangladesh and the Netherlands shows. To a large extent, then, impacts are uneven — yet because the climate system and the global economy are interconnected, even the least-exposed, wealthiest countries will not escape entirely. The clearest injustice is that those least responsible are hit hardest.