IB Geography 2.2.1 Climate Change and the Hydrosphere, Atmosphere and Biosphere Notes

Ice Stores: Only Grounded Ice Raises Sea Level

1. Sea ice floats, so it already displaces water equal to its own weight, which leaves ocean volume almost unchanged when it melts.
2. Land ice is grounded on bedrock rather than floating, so its mass is not yet displacing ocean water.
   1. Its meltwater therefore enters the ocean store for the first time, which holds even where the bed lies below sea level, as it does under much of West Antarctica.

Ice Sheets: Two Gravimetric Solutions, One Direction

1. NASA's GRACE and GRACE Follow-On gravity satellites give Greenland an average loss near 264 billion tonnes a year and Antarctica near 135 billion tonnes a year since 2002.
2. The NOAA Arctic Report Card 2025 puts Greenland's 2025 loss at 129 plus or minus 50 billion tonnes, against the 219 billion tonne annual average it gives for 2003 to 2024.

Glaciers Are Losing Mass Faster Each Decade

1. The GlaMBIE assessment in Nature (2025) estimates glacier loss of 273 ±16 gigatonnes a year from 2000 to 2023, with the 2012–2023 rate 36 percent above the 2000–2011 rate.
2. The WMO State of the Global Climate 2025 ranks the 2024 to 2025 glaciological year among the five worst reference-glacier mass balances since 1950, which matters downstream because a glacier is a seasonal water store as well as an ice store.

Sea Ice: Both Poles Sat Near Their Records in 2025

1. The NOAA Arctic Report Card 2025 records March 2025 as the lowest annual maximum Arctic extent in the 47-year satellite record.
2. The WMO gives the September 2025 Arctic minimum as 4.74 plus or minus 0.35 million square kilometres, which it ranks between the tenth and fourteenth lowest on record.
3. The Antarctic minimum of 2.06 plus or minus 0.10 million square kilometres, reached between late February and 1 March 2025, was tied for the second lowest on record.

Ocean Water Stores: Warmer Water and Extra Water Are Separate Contributions

1. About 91 percent of the surplus energy has been absorbed by the ocean on the WMO State of the Global Climate 2025 accounting, with about 5 percent by the continents, 3 percent by the cryosphere and 1 percent by the atmosphere.
2. Thermal expansion raises sea level because seawater fills a larger volume as it warms, while land ice melt adds mass without needing the water to warm.

The Shares Change Rank With the Period You Measure

1. IPCC AR6 assigns 38 percent of the 1901 to 2018 rise to ocean thermal expansion and 41 percent to glacier mass loss, with the two ice sheets and land water storage supplying the rest.
2. Over the shorter 1971 to 2018 window AR6 puts thermal expansion at 50 percent, glaciers at 22 percent, ice sheets at 20 percent and land water storage at 8 percent (SPM A.4.3), so 'expansion or ice' has no fixed answer.

Rates Since 1993, and What They Mean at a Coast

1. The rate averaged 2.65 plus or minus 0.3 mm a year over 1993 to 2011 and 4.75 plus or minus 0.3 mm a year over 2012 to 2025, taking global mean sea level about 11 cm above the 1993 baseline (WMO, 2025).
2. IPCC AR6 puts the longer rise at about 20 cm between 1901 and 2018, moving through 1.3 mm a year for 1901 to 1971, 1.9 mm a year for 1971 to 2006 and 3.7 mm a year for 2006 to 2018.

1. What a coast experiences is relative sea level, which combines the global, or eustatic, change with vertical movement of the land itself.
2. Minderhoud and colleagues, in Environmental Research Letters (2017), found the Mekong delta sinking about 1.1 cm a year from groundwater extraction, some 18 cm over 25 years.
3. That subsidence runs at more than twice the current global mean rate and close to ten times the 1901 to 1971 rate, so the delta's rising water line is only partly a climate signal.

Carbon Stores: Large Stocks, and the Flows into Them Are Weakening

1. NOAA puts the ocean's carbon store at about 50 times the amount held in the atmosphere or in the terrestrial biosphere.
   1. Those stores dwarf the annual flows between them, so a store's ranking changes far more slowly than a flow does.
2. The Global Carbon Budget 2025 estimates 2015–2024 mean sinks of 3.2 ±0.4 GtC in oceans and 2.4 ±0.8 GtC on land, together about half the 11.6 ±0.9 GtC emitted in 2024, although this compares decadal means with one year.

Three Stores, and What Is Happening to the Flow Into Each

1. The ocean store and its uptake: carbon dioxide dissolves at the sea surface, where warmer water holds less dissolved gas, so surface warming lowers the rate of uptake.
   1. The Global Carbon Budget 2025 estimates the ocean sink about 7 percent smaller than it would have been without climate change over 2015 to 2024.
2. The land store and its uptake: the Global Carbon Budget 2025 estimates the land sink, the annual flow into that store, about 25 percent smaller than it would have been without climate change over 2015 to 2024.

1. Drought and fire reverse the flow in parts of that store from uptake to release, which is why the land sink weakened through the 2023 to 2024 El Nino before recovering in 2025.

1. The stores do not change in the same direction: permafrost thaw and damaged ecosystems increase carbon release, while the ocean store keeps growing even as warmer surface water slows its uptake rate.

Ocean Acidification: Dissolved Carbon Dioxide Changes the Water's Chemistry

1. Dissolved carbon dioxide forms carbonic acid in seawater, and the acid releases hydrogen ions that lower pH and convert some carbonate ions into bicarbonate (NOAA).
2. Global surface ocean pH has fallen about 0.1 unit since the Industrial Revolution, an increase in acidity near 30 percent (NOAA, 2018).
3. Corals use carbonate ions to build calcium carbonate skeletons, so fewer carbonate ions can slow calcification and weaken reef growth (NOAA).

1. At the Great Barrier Reef, slower calcification can reduce reef growth and structural strength, which can affect marine habitats, fisheries, tourism and wave protection.

1. Acidification would proceed in a world with no warming at all, because the driver is the quantity of carbon dioxide dissolving rather than the temperature.

Extreme Weather: A Shift in the Statistics Rather than a Verdict on One Storm

1. Hot extremes have become more frequent and more intense across most land regions since the 1950s while cold extremes have done the reverse, which AR6 rates virtually certain.
2. Heavy precipitation events have increased since the 1950s over most land areas with sufficient observational coverage, which AR6 rates high confidence.

What Attribution Science Can and Cannot Say

1. Attribution compares the likelihood or intensity of an event in the observed climate against the same event in a modelled world without human influence.
2. AR6 concludes that some recent hot extreme events would have been extremely unlikely to occur without human influence.

Drought Has More Than One Definition

1. Meteorological drought
   1. Rainfall below the average for the region, which is what a climate graph on its own can show (Met Office).
2. Agricultural drought
   1. Low rainfall or dry soil reaching the point where crop growth suffers, so it can arrive without any rainfall record being broken.
3. Hydrological drought
   1. Streams, reservoirs and groundwater running low, which often lags the rainfall deficit by months.

The Southern Horn of Africa and the Rio Negro

1. World Weather Attribution found in April 2023 that droughts like the southern Horn of Africa's five consecutive failed rainy seasons since late 2020 had become about 100 times more likely, on a conservative estimate.
2. The Rio Negro at Manaus fell to a record low of 13.49 m on 17 October 2023 (NASA Earth Observatory), then to its lowest level in 122 years of records in October 2024 (Copernicus).
3. The WMO recorded significantly drier than average conditions in 2025, below the twentieth percentile of the 1991 to 2020 range, over much of south-western Asia, parts of eastern Africa and parts of Central Africa.
4. It reported separately that drought persisted in parts of the Amazon while easing in others, so the Amazon does not sit on that list.

Biome and Habitat Shifts: Ranges Track the Climate Envelope Poleward and Uphill

1. Chen and colleagues, in Science (2011), measured median shifts of 16.9 km per decade toward the poles and 11.0 m per decade upslope across hundreds of species.
2. Poloczanska and colleagues, in Nature Climate Change (2013), found marine leading edges advancing at an average 72 km per decade, far faster than the land rates in the same review.

Biome Boundaries: Treelines and Tundra Move With the Isotherm

1. Cazzolla Gatti and colleagues, in Scientific Reports (2019), measured the upper treeline at the Aktru site in the Russian Altai advancing about 150 metres upslope over the 52 years to 2017.
2. Arctic tundra greenness in 2025 was third highest in its 26-year satellite record, continuing record or near-record values since 2020 (NOAA Arctic Report Card 2025).

Coral Reefs: One Heat Threshold Crossed Repeatedly

1. Corals expel their symbiotic algae when water stays above their thermal tolerance, which removes their colour and most of their food supply.
2. NOAA confirmed a fourth global bleaching event in April 2024, then reported it as likely ending in mid-2025.
3. Bleaching-level heat stress reached 84 percent of the world's coral reef area between early 2023 and mid-2025, with mass bleaching recorded in at least 83 countries and territories (NOAA, June 2026).
4. A bleached coral is still alive, so a reef can recover where the heat stress ends before the polyps starve.

Agriculture: Heat, Water and Season Length Move Yields in Opposite Directions

1. Ortiz-Bobea and colleagues, in Nature Climate Change (2021), estimate that anthropogenic climate change has already cut global agricultural total factor productivity by about 21 percent since 1961, with reductions near 26 to 34 percent in Africa and in Latin America and the Caribbean.
2. Jägermeyr and colleagues, in Nature Food (2021), project global mean maize yields down about 24 percent by late century under the high emissions scenario SSP5-8.5.
3. The same projection has wheat yields up about 17 percent, because wheat grows in cooler regions that gain season length faster than they lose to heat.

Which Regions Gain and Which Lose

1. Where maize declines
   1. North and Central America, West Africa, Central Asia, Brazil and China.
2. Where wheat gains
   1. The northern United States and Canada, the North China Plains, Central Asia, southern Australia and East Africa.

Water Reaches the Field Differently, and Sometimes as Salt

1. The 2019 to 2020 Mekong Delta drought and saltwater intrusion reached ten of Vietnam's thirteen delta provinces, with production losses near 460,000 hectares (IFRC, 2020).
2. That delta is subsiding faster than the ocean is rising, so its salinity is not a climate signal on its own.
3. Upstream dam operation and sand extraction cut dry-season flow and sediment supply, which lets salt travel further inland.

Date Every Figure Because Climate Assessments Change

1. Give the year and the source with every number, since sea level, ice mass and yield projections are revised each assessment cycle.
2. Where a number cannot be sourced, describe the direction and the mechanism instead.