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Climate Change

The Earth's average temperature has risen by around 1.2°C since the industrial revolution. That number sounds modest. It is not. The last time the Earth was 4°C colder than today, ice sheets covered Britain and New York.

In plain English

The Earth has a thermostat. Sunlight enters the atmosphere, warms the surface, and is re-radiated as heat. Some of that heat escapes to space; some is trapped by gases in the atmosphere, water vapour, carbon dioxide, methane, and bounces back to warm the surface again. This is the greenhouse effect, and without it the Earth's average temperature would be around -18°C. Life as we know it would not exist.

The problem is not the greenhouse effect. The problem is that we are thickening the greenhouse, adding carbon dioxide and methane faster than the Earth's carbon cycle can absorb them, and the thermostat is being turned up.

Before the industrial revolution, atmospheric CO₂ concentration was around 280 parts per million (ppm). It had been roughly stable at that level for 10,000 years, the entire span of human civilisation. In 2024, it exceeded 425 ppm for the first time in at least 3 million years. Methane concentrations have more than doubled.

The result: the Earth is warming. The last decade was the hottest on record. The decade before that was the hottest before that. Seventeen of the eighteen hottest years ever recorded have occurred since 2000.

Five things to file under "wait, what?"

  • The rate of change is the problem, not just the magnitude. The Earth has been warmer before. During the Cretaceous, temperatures were 4–8°C higher and there were no ice caps. But those changes happened over millions of years, allowing ecosystems to adapt. What is happening now is happening over decades. The current rate of CO₂ increase is at least 100 times faster than any natural change in the ice core record, which extends back 800,000 years. Species cannot evolve fast enough. Ecosystems cannot migrate fast enough. Agricultural systems built over millennia cannot adjust fast enough.

  • Small average changes mean large extreme changes. A 1.2°C rise in average global temperature sounds negligible. It is warmer in the next room. But climate is a distribution, not a single number. Shifting the average by 1.2°C moves the entire distribution, increasing the frequency of extreme events at the tail. A heat event that occurred once every 50 years in pre-industrial climate now occurs approximately 4.8 times as often. At 2°C of warming, it would occur 13.9 times as often. The extremes are where the damage is.

  • The ocean has absorbed most of the heat and most of the CO₂. Around 90% of the excess heat trapped by greenhouse gases has been absorbed by the ocean, masking how much warming would otherwise have occurred in the atmosphere. The ocean has also absorbed around 25–30% of anthropogenic CO₂ emissions, becoming more acidic in the process. Ocean pH has dropped by 0.1 units since pre-industrial times, a 26% increase in acidity. This acidification threatens the organisms (corals, shellfish, plankton) that build calcium carbonate shells and skeletons, with cascading effects up the food chain.

  • The Arctic is warming four times faster than the global average. This is called Arctic amplification. As sea ice melts, the dark ocean absorbs more sunlight than the reflective ice did (lower albedo), which causes more warming, which causes more melting. This feedback loop is self-reinforcing. The Arctic is now warming at approximately four times the global average rate, and Arctic sea ice extent in summer has declined by around 40% since the 1970s. A seasonally ice-free Arctic Ocean, which would have been extraordinary in any previous human generation, is now considered likely within the next few decades.

  • We have already emitted enough CO₂ to commit to further warming even if we stopped today. The climate system has enormous thermal inertia. The oceans heat and cool slowly. There is a lag between when CO₂ is emitted and when its full warming effect is felt. The warming already "in the pipeline" from past emissions is estimated at around 0.3°C beyond what we have already experienced. Stopping all emissions today would not stop further warming immediately. It would, however, stop the warming from continuing to increase indefinitely.

The full story

The basic physics

Svante Arrhenius described the greenhouse effect mathematically in 1896, calculating accurately, using only arithmetic, that doubling CO₂ concentration would raise global temperature by around 5°C. The basic physics has been settled for over a century.

Carbon dioxide absorbs infrared radiation at specific wavelengths. When heat radiates from the Earth's surface, CO₂ molecules intercept it, vibrate, and re-emit it in all directions, including back toward the surface. More CO₂ means more interception, more re-emission, and more heat retained. This is not a model or a hypothesis. It is a measurable property of the CO₂ molecule that can be demonstrated in a laboratory.

What makes the real climate system complex is the feedbacks. Warmer air holds more water vapour, which is itself a greenhouse gas, and this amplifies warming. Melting ice reduces reflectivity, also amplifying warming. Thawing permafrost releases methane, amplifying warming further. These feedbacks are not speculation; they are observed processes. The uncertainty in climate projections is not about whether these feedbacks exist but about their precise magnitude.

Tipping points

The most concerning feature of the climate system is the existence of tipping points, thresholds beyond which a component of the climate system shifts to a new state, potentially irreversibly and potentially faster than the underlying warming would suggest.

Identified potential tipping points include the collapse of the West Antarctic Ice Sheet (which alone would raise sea levels by around 3.3 metres), the die-off of the Amazon rainforest (which currently absorbs around 2 billion tonnes of CO₂ per year), the breakdown of the Atlantic Meridional Overturning Circulation (which regulates temperature across Europe and North America), and the large-scale thawing of permafrost (which stores an estimated 1.5 trillion tonnes of carbon, roughly twice what is currently in the atmosphere).

Some scientists have argued that we may have already crossed tipping points for some of these systems. Others believe we are approaching but have not yet crossed them. The difficulty is that tipping point dynamics are, by definition, hard to predict precisely, and the consequences of getting it wrong are asymmetric.

What the scenarios look like

Current policies, if fully implemented, put the world on a trajectory for approximately 2.7°C of warming by 2100. The difference between 1.5°C and 2°C of warming, the targets of the Paris Agreement, translates to measurable differences in outcomes:

At 1.5°C: coral reefs decline by 70–90%, Arctic sea ice disappears in summer roughly once per century, sea level rise of around 0.4 metres by 2100, approximately 700 million people exposed to severe heat waves.

At 2°C: coral reefs decline by more than 99%, Arctic summer ice disappears roughly once per decade, sea level rise of around 0.5 metres, over a billion people exposed to severe heat waves.

At 3°C+: outcomes become increasingly difficult to project reliably, partly because tipping point interactions become more likely. This is the range that historical analogues struggle to illuminate. There is no period in human history at which the Earth was 3°C warmer while humans were present.

What can actually be done

The scientific consensus is clear on what is necessary: rapid, large-scale reduction in greenhouse gas emissions, primarily through decarbonising electricity generation, transport, industry, and food systems, combined with protecting and restoring natural carbon sinks (forests, peatlands, ocean ecosystems). The technologies required already exist for most of these transitions. The barriers are economic and political, not technological.

Solar and wind power are now the cheapest sources of new electricity generation in most of the world. Electric vehicle adoption is accelerating. Energy efficiency improvements are compounding. These are real, measurable changes.

They are also, at current rates, insufficient to meet the targets the science says are necessary. The gap between what countries have committed to and what the physics requires is large. Whether that gap closes in time is the central question of the coming decades.

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