In our assessment of the EEI, the focus is on the well observed period from 2005 to 2019 (see section 3). The EEI is about 460 TW or globally 0.90 ± 0.15 W m-2 (Trenberth 2022). This can be compared with the net ASR and OLR of about 240 W m-2 as an estimate of the flow-through energy. Consequently, the EEI is very small and cannot be directly discerned or measured. Nevertheless, it is very large compared with estimated direct human influences such as the total electricity generated globally (about 5.7 TW in 2018) (Trenberth 2022).

About 93% of the extra heat from the EEI ends up in the ocean as increasing ocean heat content (OHC). In 2022, the global OHC was the highest on record (Cheng et al 2022) and the global warming signal in OHC is large compared with the natural variability, unlike GMST, so that trends in OHC can be detected in four years (Cheng et al 2018), see figure 2 for example. The second-best signal-to-noise ratio is in the related sea level rise (SLR), as about 40% comes from OHC and the associated expansion of the ocean, while the rest mainly comes from melting of land ice: glaciers, Greenland, Antarctica that puts more water into the ocean. For SLR the trend detection occurs in about five years while for GMST the trend detection requires more than two decades (Cheng et al 2018).

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Figure 2. For 2000–2021, GMST as in figure 1, and corresponding OHC in zettajoules relative to a baseline mean for 1981–2010, based on Cheng et al (2022). Click the image to enlarge.

On average nearly 3% of the EEI goes into melting ice and another 4% goes into raising temperatures of land and melting permafrost, while less than 1% remains in the atmosphere. The changes in GMST relate to those on land and sea surface temperatures (SSTs) in the ocean, and the increase in GMST (figure 1) is a consequence. The primary reasons for these distributions of excess heat relate to the heat capacity of these climate system components, the specific heat of water versus land versus air, and the masses involved; see Trenberth (2022) for a comprehensive review.

However, the initial excess energy has profound effects and big impacts along the way to its destination. The radiative energy is mostly absorbed at the surface where it mainly contributes to increased evaporation and moistening of the atmosphere, and is eventually realized as latent heating of the atmosphere during precipitation. This invigorates the hydrological cycle. In regions where it is not raining, it leads to enhanced drying then heating, increasing risk of heat waves and wildfires on land. It increases intensity of droughts. The evaporated moisture, carried by the atmosphere, converges into storms and associated frontal systems, where it increases precipitation rates, increasing risk of flooding on land. The latent heat released may also intensify some weather systems such as hurricanes and convection (thunderstorms). The warmer moister air is more buoyant and, helped by many weather phenomena, it rises and expands in lower pressures and cools adiabatically, then spreads out maybe thousands of km away and warms as it subsides, thus redistributing energy often to higher and drier regions where it can radiate to space.

In general, all components of the climate system react to heating by trying to get rid of excess heat in one way or another. The most effective method overall globally is radiative cooling as higher temperatures increase radiation by the fourth power of absolute temperature. Moreover, unequal heating leads to gradients that drive instabilities in the atmosphere (convective, baroclinic) and mixing is pervasive. In the ocean, warmer waters and those with lower salinity are less dense, and thermohaline circulations may develop, although most ocean currents are driven by winds. In ice and land, energy may be redistributed by water flows or conduction, but the latter is a very inefficient process.

In the ocean, heating occurs from the top down; warm on top of colder water is a stable configuration so that the stability and stratification of the ocean increase (Li et al 2020). Whereas globally, GMST and SSTs have clearly increased since about the 1970s (figure 1), for deeper ocean layers there is a delay that increases with depth. Globally, the top 500 m of the ocean are clearly warming since 1980, for 500–1000 m depth since 1990, 1000–1500 m layer since 1998, and from 1500 to 2000 m since 2005 (Cheng et al 2021). Indeed, it is a major challenge for climate models to get this heat penetration right, since it depends on unresolved sub-grid scale processes like mixing and convection, and how well or whether tidal mixing is included. A preliminary check of CMIP6 models reveals too much heating of the deep ocean (too large diffusion) but too little heating of the layers given above. This ocean heat redistribution is a factor in climate sensitivity of models and the errors contribute to too much surface warming.

The GMST was the fifth or sixth warmest on record in 2021 (depends on the dataset), in part, because of the year-long La Niña conditions, in which cool conditions in the tropical Pacific influence weather patterns around the world. Under those conditions the ocean stores extra heat, and then releases heat during El Niño events, making El Niño years like 1998 and 2016 relatively the warmest on record while OHC declines a small but measurable amount. There is also a signal in SLR because more rain occurs over the ocean in El Niño and droughts are more common over land, while more rains and snows occur on land in La Niña events. In the La Niña event in 2011 the extensive rains and snows, especially over North America, Siberia, and Australia (where it led to revival of Lake Eyre) took 5 mm of water out of the global ocean (Boening et al 2012).

There is a lot more natural variability in surface air temperatures than in ocean temperatures because of El Niño/La Niña and weather events (figure 2). For the oceans, the natural variability on top of warming creates hot spots locally, sometimes called 'marine heat waves,' that vary from year to year but are increasing (Tanaka et al 2022). Those hot spots have profound influences on marine life, from tiny plankton to fish, marine mammals, and seabirds. Other hot spots eventually result in more activity in the atmosphere, such as hurricanes (Trenberth et al 2018) which then take heat out of the ocean even as some is also mixed deeper.

All five oceans are warming (Cheng et al 2022), with the largest amounts of warming in the Atlantic Ocean and Southern Ocean surrounding Antarctica. That is a concern for Antarctica's ice as warmer waters can creep under Antarctica's ice shelves, thinning them and resulting in calving off huge icebergs.