The ocean is warning us about the climate and the signs are undeniable

We need to talk about the weather and climate, again. As I and some fellow climate scientists have discovered, in research published in the Journal of Climate, the warming of the world’s oceans is proceeding at different paces in different latitude bands and New Zealand is in a region where some of the biggest changes are occurring in the global ocean. 

This affects the atmospheric winds and weather patterns, which in turn change ocean currents and where the heat piles up in the ocean. Often warmer waters can influence the locations of fish and the health of marine ecology (such as kelp farms and phytoplankton). This will affect our local oceanic ecosystems.

Our research involved producing maps and graphs of changes in incoming solar radiation at the top-of-atmosphere, and outgoing infrared radiation to space beginning in March 2000. We have also computed all changes in pressure, winds, temperatures, and humidity at all points in the atmosphere hourly, from 2000 to 2023. 

This meant we could compute the movement of atmospheric energy—heat, kinetic energy, potential energy, latent energy (moisture)—and changes over time. We also processed ocean observations after 1958 (the International Geophysical Year) when adequate data became available to map heat changes in the ocean down to a depth of 2km.

Change from human activities is often called global warming because the main human influence is from changes in the atmospheric composition through the burning of fossil fuels that dump carbon dioxide, a greenhouse gas, into the atmosphere. That is, it traps heat that would normally be lost back to space and therefore increases temperatures at the surface.

Moreover, carbon dioxide has a very long lifetime, even as it is recycled through photosynthesis in plants, and quickly becomes globally distributed. Carbon dioxide has increased since the 1800s by over half, and the rate of increase has been greatest in the last five years. It would be more accurate to call it global heating rather than warming.

About 70% of the planet is ocean and, unlike the land, it soaks up heat. Heat slowly penetrates down and can now be tracked to well below depths of 2km. 

Surface hot spots or marine heat waves in the ocean are not uncommon and arise from favourable weather (light winds, sunny), but are now becoming pervasive. Our new study shows that these are changing weather patterns, and the sea level is rising by over 4mm a year.

Warming oceans

Our new research has uncovered rather distinctive patterns of change and why they are occurring. The changes over the last 25 years in the ocean relate strongly to the normal distribution of mid-latitude westerlies and tropical easterly trade-winds. Together these push waters to pile up from 30° to 50° latitude over the oceans (see graph below).

Global mean Ocean Heat Content for 0-2000m relative to a base period 1981-2010, in ZettaJoules. The 95 percent confidence intervals are shown. Right: Trend from 2000-2023 in OHC for 0-2000m in W m-2. The ringed regions highlight the biggest increases.

The greatest heating is near 40°N for northern oceans, and 40° to 45°S over the southern hemisphere—the latitudes of New Zealand. In turn the change in sea temperatures alters the atmosphere. Hence the atmosphere and ocean currents are systematically redistributing heat from global warming, profoundly affecting local climates. Our study shows the jet streams and associated storm tracks over the oceans in both hemispheres have shifted polewards. 

It never rains but it pours

The atmosphere (the mixture of gases around the Earth) can hold about 7% more moisture for every 1°C increase in temperature. Most moisture naturally occurs where temperatures are higher: near the surface, in the tropics and subtropics, and where surface waters (sea, lakes, etc) provide ample sources of moisture.

The first order effect of the warmer ocean is greater evaporation of moisture, increasing atmospheric humidity. Global observations reveal over 7% increases in total column water vapour in the atmosphere since 2000. The increased moisture is then caught up in weather systems and so it rains harder than it otherwise would have. 

This is also likely to intensify a storm through the release of the latent heat that went into the evaporation in the first place. The result is more extreme atmospheric rivers and weather, and increased risk of flooding, as scientists are observing and the whole world is experiencing.

Heating generally produces increased surface drying as water is sucked up into the thirsty atmosphere, and where it is not raining it leads to drought, heat waves and wildfires. Whether wildfires are a consequence depends on how well forests and other areas are managed.

But atmospheric moisture and clouds are carried around by winds and get caught up in storms, including hurricanes/cyclones and atmospheric rivers. This can result in flooding rains, as happened in Auckland on January 27th 2023 (over 280mm of rain in a day), and Cyclone Gabrielle in mid-February (widespread flooding and erosion in eastern North Island). Whether heavy rains result in flooding depends, of course, on drainage systems and surface water management, and whether people have settled in floodplains, which is so often the case as populations have exploded.

The influence of La Niña in 2025 has meant extra rains for Australia, such as Cyclone Alfred in March 2025 that brought widespread flooding near Brisbane. As warm waters developed around New Zealand in February and March 2025, we experienced long dry spells and great holiday weather. But it catches up sooner or later, with strong damaging winds and heavy rains from Ex-Cyclone Tam April 16-19. 

Satellite imagery from the Himawari (Japanese geostationary satellite) at 2200 UTC 17 April 2025 (10 a.m. 18 April New Zealand time)

The changes in winds have been shown to be in large part caused by changes in the atmospheric composition. There is a lot of speculation about changes in aerosols (pollution) and related changes in cloud, especially from clean-up of use of high-sulphur coal by ships in the North Atlantic and cleaner air from China, but these effects are not in play in the southern hemisphere. Rather the warming oceans and changes in atmospheric circulation are altering the ways the climate system works, with consequences for where storms and rains occur, and hence risk of flooding and erosion. 

Atmospheric heatwaves and marine heatwaves in and around New Zealand have increased to give compound climate extremes, with melting glaciers in the South Island.

How much more evidence (both scientific and lived experience) do we need to recognise that these are linked and plan accordingly?

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