Climate Change and Hazard Response

🔍 Evidence That Climate Is Changing

One of the most important things to understand about climate change is that scientists do not rely on a single line of evidence. If only temperature records existed, there would be room for doubt — instruments might be poorly placed, records might be incomplete. But climate scientists have assembled multiple independent lines of evidence, each measuring something completely different, and they all tell the same story. This convergence of evidence across different fields and methods is why the scientific consensus on climate change is so strong.

1. Temperature Records

The global average surface temperature has risen approximately 1.2°C above pre-industrial levels (the 1850–1900 baseline) by 2023. The ten hottest years on record have all occurred since 2010. The year 2023 was the hottest single year ever recorded at 1.45°C above the pre-industrial baseline. These measurements come from thousands of weather stations on land and thousands of ocean buoys, independently operated by different national agencies — making coordinated error or fraud essentially impossible.

2. Ice Cores

Ice cores drilled from the Antarctic and Greenland ice sheets provide the most direct window into Earth's ancient climate. Layers of ice form each year like tree rings, trapping tiny air bubbles containing samples of the atmosphere at the time. Scientists can drill down and extract air from 800,000 years ago. These records show that CO₂ concentrations have fluctuated between roughly 180 ppm (ice ages) and 280 ppm (warm interglacials) over the past 800,000 years — and that temperature closely tracks CO₂ throughout. Since industrialisation, CO₂ has risen to 421 ppm in 2023 — a level not seen for at least 3 million years, far outside the natural range of variation. The rate of increase is also unprecedented: CO₂ is rising about 100 times faster than at the end of the last ice age.

3. Sea Level Rise

Global sea levels have risen by approximately 21–24 cm since 1880. The rate of rise has accelerated markedly: satellite data since 1993 shows a current rate of 3.7 mm per year, roughly double the average rate of the 20th century. Sea level rises for two interconnected reasons: thermal expansion (warmer water physically expands and takes up more volume) and melting ice (glaciers, the Greenland ice sheet, and parts of the Antarctic ice sheet are adding water to the ocean). The Greenland ice sheet alone contains enough water to raise global sea levels by approximately 7 metres if it melted entirely.

4. Glacier Retreat

Glaciers worldwide are retreating at accelerating rates. The Rhône Glacier in Switzerland lost 1.4 km in length in just 20 years (2000–2020) and has retreated more than 3 km since records began. Arctic sea ice extent has declined by roughly 13% per decade since satellite monitoring began in 1979; Arctic summer sea ice has lost approximately 40% of its area. The Himalayan glaciers — which supply freshwater to over a billion people across South Asia — are losing mass faster than almost anywhere else on Earth. The IPCC projects that two-thirds of Himalayan glacier volume could be lost by 2100 under high-emissions scenarios.

5. Shifting Seasons and Phenological Changes

Phenology — the study of seasonal biological events such as flowering, migration and hibernation — provides a biological record of warming. In the United Kingdom, spring is arriving approximately 26 days earlier than in the 1980s, based on records of first flowering dates kept by amateur recorders over centuries. Migratory bird species are arriving earlier and departing later. In the ocean, species distributions are shifting poleward as water temperatures rise.

6. Coral Bleaching

Coral reefs are among the most temperature-sensitive ecosystems on Earth. When water temperatures rise just 1–2°C above the normal summer maximum, corals expel the symbiotic algae (zooxanthellae) that give them colour and provide up to 90% of their energy — a process called bleaching. If temperatures remain elevated, corals starve and die. The Great Barrier Reef experienced four mass bleaching events between 2016 and 2024, with over 50% of the reef bleached in 2022 — a frequency that gives individual reefs no time to recover between events.

🌍 Natural vs Human Causes of Climate Change

A critical point for the exam: climate has always changed naturally. The Earth has experienced ice ages and warm periods throughout its 4.5-billion-year history — long before humans existed. You must know the natural causes of climate change for the exam. However, natural causes cannot explain the current rate or pattern of warming. The current phase of warming is happening far faster than any natural process can account for, and the scientific evidence — summarised in the IPCC Sixth Assessment Report (2021) — concludes with "unequivocal" certainty that human influence is the dominant cause of warming since 1950.

Natural Causes

Human Causes

Human causes have dominated the warming signal since approximately 1950, and are almost entirely responsible for warming since the 1970s. They work by increasing the concentration of greenhouse gases in the atmosphere, enhancing the natural greenhouse effect.

⚙️ The Enhanced Greenhouse Effect — The Mechanism

Figure 1: The Enhanced Greenhouse Effect. Human emissions of CO₂, methane and nitrous oxide trap more outgoing heat, raising global temperatures above natural levels.

Not to scale. Some long-wave radiation passes through to space. The rest is absorbed by greenhouse gases and sent back in all directions, including down to the surface.

To understand climate change, you must understand the distinction between the natural greenhouse effect and the enhanced greenhouse effect.

The Natural Greenhouse Effect

Without any greenhouse effect, Earth's average surface temperature would be approximately -18°C — far below freezing. The actual average is approximately +15°C. This difference of 33°C is entirely due to the natural greenhouse effect, which has existed throughout Earth's history and makes the planet habitable. Here is how it works:

The Enhanced Greenhouse Effect

Human activities — primarily burning fossil fuels and deforestation — have increased the concentration of greenhouse gases in the atmosphere far above their natural levels. More greenhouse gas molecules mean more infrared radiation is absorbed and re-radiated back to Earth. The planet's energy budget becomes unbalanced: more energy arrives from the Sun than can escape back to space. The result is a gradual build-up of heat in the Earth system — measured as rising surface temperatures, warming oceans, and melting ice.

Key Greenhouse Gases and Their Warming Potential

The long atmospheric lifetime of CO₂ means that emissions today will continue to trap heat for centuries. Even if all human CO₂ emissions stopped tomorrow, the warming already "locked in" from past emissions would continue for decades. This is why early action on mitigation is so much more effective than waiting.

🌊 Effects of Climate Change

Climate change produces effects at every scale — from the melting of polar ice to changes in crop yields in individual farming communities. In the exam, you will gain higher marks by organising effects under clear headings and by connecting physical changes to their human consequences. The strongest answers recognise that physical and human effects are deeply linked: a rise in sea level is a physical change, but it becomes a human catastrophe when millions of people lose their homes and livelihoods.

Physical Effects

• Sea level rise — a slow-moving catastrophe: Global mean sea level has risen 21–24 cm since 1880 and is currently rising at 3.7 mm per year. Projections suggest a further 0.3–1 m by 2100 under current trajectories, with higher estimates possible if large ice sheet instabilities are triggered. Even modest rises threaten enormous populations: Bangladesh has approximately 17 million people living within 1 metre of sea level; the Maldives has an average land elevation of just 1.5 m above sea level; the Pacific island nation of Tuvalu is purchasing land in Fiji in anticipation of becoming uninhabitable. Coastal cities including Miami, Jakarta, Mumbai and Shanghai face growing flood risk.
• More intense storms: Tropical storms (hurricanes and typhoons) draw their energy from warm ocean water. As sea surface temperatures rise, the maximum potential intensity of tropical storms increases. While the total number of tropical storms may not change significantly, the proportion reaching the highest categories (4 and 5) is projected to increase. The 2017 Atlantic hurricane season — which included Harvey, Irma and Maria — caused approximately $300 billion in damage, part of a trend of rising economic losses from extreme weather.
• Drought and water stress: Climate change is projected to intensify the global water cycle — wet areas getting wetter, dry areas getting drier. The Mediterranean region (southern Europe, North Africa, Middle East) is one of the most drought-affected areas, with rainfall declining and temperatures rising significantly faster than the global average. California experienced its driest three-year period on record (2020–2022) before catastrophic flooding in 2023 — demonstrating how climate change increases the swings between extremes as well as the averages. The Sahel in sub-Saharan Africa faces increasingly severe and prolonged droughts, threatening food and water security for hundreds of millions of people.
• Flooding and extreme rainfall: While droughts increase in some regions, other areas face greater flood risk as a warmer atmosphere holds more water vapour and produces heavier rainfall events when it does rain. The UK experienced unprecedented flooding in 2020 (Storm Dennis) and 2021 (flooding in London's underground). Germany and Belgium experienced catastrophic flash flooding in July 2021, killing over 200 people in events that scientists attributed partly to climate change. Thawing permafrost in Arctic regions releases methane (a more potent greenhouse gas than CO₂), creating a positive feedback loop that accelerates warming.
• Ecosystem disruption: Coral bleaching has been described above. Polar bears require sea ice to hunt seals — declining Arctic sea ice is reducing their habitat. Phenological mismatches are emerging: some species are responding to warming by advancing their seasonal behaviour (e.g. plants flowering earlier), while other species in the same ecosystem are not, disrupting food webs. Species are migrating poleward and to higher altitudes as temperatures rise — but not all species can do so, and many face extinction.

Human and Economic Effects

• Food security: Crop yields are projected to decline in many tropical and subtropical regions as temperatures rise beyond optimal growing conditions. The IPCC projects that climate change could reduce crop yields for staple foods (wheat, rice, maize) by 2–6% per decade in a 2°C warming scenario. At the same time, some higher-latitude regions (Canada, Russia, Scandinavia) may see increased yields as temperatures warm. The net effect is an increase in global food insecurity, concentrated in the countries that are already most vulnerable.
• Human migration: The World Bank projects that climate change could force 216 million people to migrate internally within their own countries by 2050 in a business-as-usual scenario, as water stress, crop failures, sea level rise and coastal flooding make their home regions unliveable. This represents one of the largest forced migration events in human history — with profound implications for geopolitical stability, urban infrastructure and international relations.
• Health: Rising temperatures increase heat stress mortality directly; global heat-related deaths have increased significantly in the 21st century. Warmer temperatures are also expanding the range of vector-borne diseases: malaria (carried by Anopheles mosquitoes) is spreading to higher altitudes in East Africa and South Asia as temperatures permit mosquito survival. Dengue fever is expanding its range significantly. Air quality deteriorates in heat waves as ground-level ozone forms more readily.
• Economic damage: Global insured losses from natural disasters reached approximately $100 billion in 2023 — and uninsured losses were considerably higher. Developing countries tend to bear a disproportionate share of climate-related economic losses relative to their GDP. The economic cost of inaction on climate change is projected to significantly exceed the cost of mitigation: the Stern Review (2006) estimated that unmitigated climate change could reduce global GDP by 5–20%, while the cost of stabilising emissions would be approximately 1% of GDP per year.

🌱 Responses to Climate Change: Mitigation

Mitigation means reducing the causes of climate change — cutting greenhouse gas emissions or enhancing the natural systems (forests, soils, oceans) that absorb CO₂. Mitigation strategies address the root of the problem: if successful, they slow or halt the rise in atmospheric CO₂ and reduce the eventual scale of warming. They require action at every scale — individual, national and international — and are politically challenging because their benefits are global and long-term while their costs are immediate and national.

Mitigation strategies vary enormously in scale, cost and feasibility. International agreements like the Paris Agreement set the overall framework but rely on voluntary national action. Technological solutions like CCS exist but remain expensive and limited in scale. The political challenges of mitigation — particularly the mismatch between countries that have emitted most historically (wealthy industrialised nations) and countries that are most vulnerable to climate impacts (poorer, tropical nations) — remain one of the most difficult obstacles to global action.

🏗️ Responses to Climate Change: Adaptation

Adaptation means adjusting how societies live and work to cope with the effects of climate change that are already happening or unavoidable. Even if all human emissions stopped tomorrow, some warming and its consequences are already locked in. Adaptation does not address the cause of climate change — it manages the consequences. It is generally more immediately achievable than global mitigation but does not reduce the underlying risk for future generations. The exam regularly asks you to compare HIC and LIC adaptation strategies, emphasising how wealth determines the capacity to adapt.

The contrast between Bangladeshi and Dutch adaptation strategies illustrates a fundamental principle: adaptation capacity is closely linked to development level. The Netherlands can invest billions in engineered flood management because it has a GDP per capita of approximately $55,000 and a world-leading tradition of water engineering. Bangladesh, with GDP per capita of approximately $2,500, relies on lower-cost community-based and agricultural approaches. The Maldives, despite being a middle-income country in relative terms, faces existential risk from sea level rise regardless of wealth. This raises profound ethical questions about climate justice: the countries most vulnerable to climate change are often those that have contributed least to causing it.