14.3 — What Warming Actually Does

A global average temperature rise of 1.5 or 2 degrees sounds trivial. Most people experience larger swings between morning and afternoon.

The number is misleading in three specific ways, and understanding why is the whole point of this chapter.

It is a global average of a system that varies enormously. Land warms faster than ocean. High latitudes warm several times faster than the tropics. The Arctic has warmed by around 3 to 4 °C.

It is an average of a distribution, and the damage is in the tails. Shifting a distribution of daily temperatures a small amount to the right multiplies the frequency of the extreme values at the end — a modest shift in the mean can produce a several-fold increase in the number of days above a damaging threshold.

And 1.5 °C globally is a very large change historically. The difference between now and the depth of the last ice age, when a three-kilometre ice sheet sat on Canada (Chapter 2.8), is around 5 to 6 °C. We are talking about a fifth to a third of an ice age, in the other direction, in about two centuries.

Heat

The most direct effect and the one where India is most exposed.

Chapter 2.3 gave the physiology. The body sheds heat by evaporating sweat, and evaporation slows as humidity rises. The combined measure is the wet-bulb temperature.

A sustained wet-bulb temperature above roughly 35 °C is unsurvivable for a healthy person even at rest in shade, because there is no route left to dump metabolic heat. Serious danger begins well below that, particularly for outdoor workers, the elderly and anyone with cardiovascular disease.

Brief excursions near that threshold have already been recorded in the Persian Gulf and in parts of the Indus and Ganges plains.

And the exposure is concentrated among people with no air conditioning who work outdoors — construction workers, farm labourers, delivery workers, street vendors. Heat is an inequality problem before it is a mortality problem.

The measurable Indian consequences. Heat-related deaths have been recorded in the thousands in individual severe events. Labour productivity in outdoor work falls sharply above certain thresholds, with estimated losses of many billions of working hours annually across South Asia. And the 2022 spring heatwave, which arrived in March, cut Indian wheat yields enough to prompt an export restriction.

What works. Ahmedabad's heat action plan, adopted after the 2010 heatwave, is the standing case: early warning, public cooling spaces, adjusted working hours, hospital preparation, and cool roofs. Independent assessment estimated it prevented over a thousand deaths a year. Dozens of Indian cities have since adopted versions.

Water

More warming means more evaporation and a wetter atmosphere (Chapter 2.3) — around 7 percent more water vapour per degree.

Which produces a pattern rather than uniform wetness: wet places get wetter, dry places drier, and rainfall arrives in fewer, heavier events.

Heavy rainfall events are increasing in frequency and intensity across most regions, and this is one of the better-established projections.

For India specifically. Total monsoon rainfall projections remain uncertain (Chapter 2.5). What is better established is a shift toward more intense rainfall concentrated in fewer days, with longer dry spells between — which is the worst configuration for agriculture and for drainage, because the same annual total delivered in fewer events means more runoff, less infiltration, more flooding and less soil moisture.

Glacier retreat. Himalayan glaciers are losing mass, which affects the dry-season flow of the Indus, Ganges and Brahmaputra (Chapter 1.8). The commonly repeated claim that they will disappear by 2035 was an error in a 2007 assessment report and was corrected; the actual projections are slower and are still serious. Glacial lake outburst floods — a lake dammed by moraine bursting — are an increasing hazard, and the 2021 Chamoli and 2023 Sikkim disasters are recent instances.

Groundwater (Chapter 1.12) is being depleted independently of climate and is made worse by it, since less reliable surface water means more pumping.

Food

Effects run in both directions and the balance is negative.

Positive: longer growing seasons at high latitudes, some regions becoming cultivable, and the carbon fertilisation effect, which raises yields.

And the carbon fertilisation effect has a documented catch. Crops grown at elevated carbon dioxide concentrations show reduced concentrations of protein, zinc and iron — measured in field experiments across wheat, rice and other staples. More calories and less nutrition per calorie, which matters most where diets are already marginal.

Negative and larger: heat stress during the grain-filling stage, which reduces yield sharply; water stress; more variable rainfall; expanded pest and disease ranges; and, for rice, sensitivity to night-time temperatures.

Estimates for South Asian yields under continued warming are consistently negative for wheat and mixed to negative for rice.

Sea level

Two contributions. Thermal expansion — water expands as it warms — and melting land ice.

Sea level has risen around 20 centimetres since 1900 and the rate has roughly doubled since the early 1990s.

Projections to 2100 depend on emissions and on ice sheet behaviour, and the plausible range runs from around 0.3 metres at the low end to over a metre at the high end, with higher values not excluded if ice sheet processes turn out to be faster than currently modelled.

And the important point is that it does not stop in 2100. Sea level responds to warming over centuries, so a given amount of warming commits the world to a rise that continues long after temperatures stabilise.

What it means practically. Coastal flooding that was a once-a-century event becomes annual in many places. Storm surge (Chapter 2.4) rides on a higher baseline. Saltwater intrusion contaminates coastal groundwater and agricultural land, which is already a serious problem in the Sundarbans and in coastal Gujarat and Tamil Nadu.

India's exposure. Around 7,500 kilometres of coastline. Mumbai, Chennai, Kolkata, Kochi and Visakhapatnam are all exposed. The Sundarbans delta is losing islands, and populations have already moved. Bangladesh, on the same delta, is among the most exposed countries in the world.

Storms

Chapter 2.4 gave the physics. Warmer sea surface means more available energy; warmer air holds more water.

What the evidence supports. A trend toward a higher proportion of intense storms, higher rainfall rates, and higher storm surge from the sea level baseline. Rapid intensification — a storm strengthening very quickly before landfall, which is the worst case for evacuation — appears to be becoming more common.

What is not established. A clear trend in the total number of tropical cyclones.

Ecosystems

Coral reefs are the most immediately threatened. Bleaching occurs when warm water causes corals to expel their symbiotic algae; repeated bleaching without recovery time kills the reef. Major global bleaching events have occurred with increasing frequency. Combined with acidification (Chapter 1.11), projections for reefs at 1.5 °C of warming involve losses of most of them, and at 2 °C nearly all.

Species ranges are shifting poleward and upslope, and species that cannot move fast enough or have nowhere to go — mountain-top and polar species particularly — are at greatest risk.

And timing mismatches. If insects emerge earlier but migratory birds arrive on their old schedule, the food is gone when the chicks hatch. These decouplings are documented across many systems.

Tipping points

Elements of the system that may shift into a different state that does not reverse when the forcing is removed.

The candidates. The Atlantic overturning circulation (Chapter 2.6). The West Antarctic and Greenland ice sheets. Amazon dieback, where reduced rainfall converts forest to savanna, which further reduces rainfall. Permafrost carbon release (Chapter 14.1). And coral reef systems.

The honest state of the science. Their existence is well supported by palaeoclimate evidence — Chapter 3.6's Younger Dryas is a documented abrupt shift. Where the thresholds are is poorly constrained. Recent assessments suggest several may have thresholds within the 1.5 to 2 °C range, which is a reason for concern and is not a prediction of imminent collapse.

And the reason they matter for policy is asymmetry. A low-probability outcome with an irreversible catastrophic consequence justifies more precaution than the expected value alone suggests — which is the same reasoning as buying insurance.

Who bears it

The distribution is the moral centre of the subject.

Cumulative emissions. The United States and Europe account for a large share of historical emissions. India's per capita emissions are around a third of the world average and roughly a seventh of American ones, and its cumulative share is small relative to its population.

Current emissions. China is the largest annual emitter, the United States second, India third by volume and far lower per person.

And exposure. Tropical and low-lying countries face the largest physical impacts, and they are disproportionately the countries with the smallest historical contribution and the least capacity to adapt.

This is the basis of the common but differentiated responsibilities principle in the climate treaties, and it is the reason India's negotiating position has consistently linked emission commitments to development space and to finance.

The loss and damage fund, agreed in principle in 2022, is the institutional response and its scale relative to the estimated costs is very small.

What is already locked in

Because of the residence time (Chapter 14.1) and the ocean's thermal inertia, some further warming is committed regardless of what happens next.

Which means adaptation is not an alternative to mitigation. It is a requirement.

And adaptation works. Chapter 2.4's cyclone death tolls fell from around 10,000 in 1999 to under 50 in 2013 in Odisha, through forecasting, shelters and evacuation. Ahmedabad's heat plan works. Flood-tolerant rice varieties exist and are being adopted. Early warning systems, building codes, drainage, mangrove restoration and crop diversification all reduce damage measurably.

Adaptation is cheaper than the damage it prevents, by wide margins in most assessments, and it is systematically underfunded relative to mitigation.

Where this shows up in your life

Insurance premiums in flood and cyclone-exposed areas, which are rising and in some places becoming unavailable, which is the market pricing risk before the politics does.

Food prices, which respond to harvest failures anywhere in a globally traded system.

And, if you are in India, heat. The single most useful practical knowledge from this chapter is the wet-bulb point: it is the combination of heat and humidity that kills, not the temperature alone, and 38 °C at high humidity is more dangerous than 45 °C in dry air. Volume V's emergency chapters cover recognition and treatment of heat illness.

What the next page covers

Chapter 14.4 assesses every energy source honestly — coal, oil, gas, nuclear, hydro, solar, wind, biomass and the rest — on cost, on emissions, on land use, on deaths per unit of energy, and on the practical problems each has, including the ones its advocates do not mention.