Water, carbon, volcanoes and climate trends

Water in the atmosphere

• About 86% of the water that evaporates each year comes from the oceans. Plants add more by transpiration, and the vapour is carried by the winds, rises, cools, condenses into cloud and falls as precipitation.
• A water molecule stays in the atmosphere for only about 9 days on average, so the air holds little water at any moment but moves a huge amount through each year.
• Evaporating water absorbs latent heat, about 2.26 × 10⁶ J for every kilogram, without changing temperature; condensation releases it into the air, often thousands of kilometres away. This latent heat is one of the main ways heat moves from the surface and from the tropics into the atmosphere.
• Long, narrow bands of very moist air called atmospheric rivers carry tropical water vapour towards the mid-latitudes. When one reaches New Zealand and is lifted by the ranges, it can drop extreme rain, as in many West Coast floods and Auckland's January 2023 floods.

Try it: the water cycle

Carbon in the atmosphere

• Carbon moves into the air through respiration, decomposition, fires, volcanoes, release from warm ocean water, and burning fossil fuels; it leaves through photosynthesis and by dissolving in cold ocean water (the physical pump), with marine plankton then carrying some to the deep ocean (the biological pump).
• Without people these flows roughly balanced. Burning fossil fuels and clearing forest now add about 11 billion tonnes of carbon a year; roughly half stays in the air, about a quarter dissolves in the ocean and about a quarter is taken up by land plants.
• CO₂ is well mixed, so measurements at Mauna Loa in Hawaiʻi and Baring Head near Wellington show the same rising trend.
• Each year the record wiggles: CO₂ falls during the northern summer, when the large forests of the Northern Hemisphere photosynthesise, and rises in the northern winter. The wiggle is much smaller at Baring Head, far from those forests.

Try it: the CO₂ record

Volcanic gases and aerosols

• Eruptions release ash, water vapour, carbon dioxide and sulfur dioxide (SO₂). What happens next depends on how high the column reaches.
• In the troposphere, ash falls out within hours to days and SO₂ forms sulfate that rain washes out within about a week or two, so effects are local and short: haze, acid rain and cooler days under the plume.
• An explosive eruption can push SO₂ into the stratosphere, where there is no rain. Over weeks it reacts with water to form tiny sulfuric acid droplets, which stratospheric winds spread around the globe within months.
• This aerosol layer reflects sunlight back to space, cooling the surface for one to three years until the droplets slowly settle out; it also absorbs heat, warming the stratosphere. After Pinatubo (Philippines, 1991) global surface temperature fell by about 0.5 °C in 1992.
• Volcanic CO₂ is small compared with human emissions, and volcanic cooling is temporary, so eruptions cause dips in the warming trend rather than reversing it. The 2022 Hunga Tonga-Hunga Haʻapai eruption was unusual: it sent about 150 million tonnes of water vapour, a greenhouse gas, into the stratosphere along with relatively little SO₂.

Global trends

• Global surface temperature in 2011 to 2020 was about 1.1 °C above 1850 to 1900, and 2024 was the warmest year on record, about 1.55 °C above.
• Land warms faster than ocean, and the Arctic is warming fastest, because melting snow and ice expose darker surfaces that absorb more sunlight.
• Air holds about 7% more water vapour for every 1 °C of warming, so heavy rain events are becoming more intense, while subtropical regions tend to get drier: wet regions wetter, dry regions drier.
• Climate is the long-term average of weather, usually over 30 years, so trends are found by comparing averages or fitting a line, not from one hot day or one cold winter.

New Zealand's climate trends

• NIWA's seven-station series, from long-running sites around the country, shows New Zealand's average temperature rose by about 1.1 °C between 1909 and 2019.
• Since 2013 New Zealand has had a run of its warmest years on record, with 2022 the warmest, and there are fewer frosts and more hot days than in the mid-1900s.
• Rainfall trends differ by region: generally more rain in the south and west of the South Island and less in parts of the north and east, matching stronger westerlies and the drying of the subtropics.
• Southern Alps glaciers have lost a large share of their ice since the late 1970s, measured each year in NIWA's end-of-summer snowline survey.

What Achieved, Merit and Excellence look like

• Excellence answers connect processes across the system: where the gas goes, what it does to energy, and why the effect lasts as long as it does.