1.4.1 Introduction

The Earth’s ocean and atmosphere form the flowing fluid parts of the Earth system that circulate around the planet. They drive the daily weather and climate patterns we see. On a global scale, the dominant circulations in the atmosphere are a consequence of regional differences in solar radiation (with poles less heated than the equator), Earth’s rotation (redirecting winds) and thermodynamic properties (e.g. that warm air is less dense and rises). 

Atmospheric circulation can be divided into several rotating cells: The ‘Hadley cell’ is formed either side of the equator by warm air rising near the equator (at the ‘Intertropical Convergence Zone’, or ITCZ) before sinking in both mid-latitudes (at ~30° North or South). The mid latitude Ferrel cell sinks at mid latitudes and rises at high latitudes (~60° N or S), connecting to the polar cell rising at high latitudes and sinking at the poles. Diverted by Earth’s rotation, surface winds tend to blow westwards (the ‘easterly’ trade winds) in the tropical cells, and eastwards (‘westerlies’) in the mid and high latitudes.

Over 70 per cent of the Earth’s surface is covered by the global ocean, and is conventionally divided into the Atlantic, Indian, Pacific and Southern oceans. Ocean currents circulate water around the Earth as a result of pressure gradients driven by differences in temperature and salinity. This ‘global thermohaline circulation’, also known as the ‘ocean conveyor belt’, mixes the whole ocean over a roughly thousand-year timescale. Key components of this mechanism, connecting deep currents with those on the surface, are the sinking of cold and salty – therefore dense – water in polar regions as well as widespread ‘upwelling’. The force exerted by atmospheric surface winds leads to basin-wide rotating ‘gyres’ of surface currents in the various ocean basins (Figure 1.4.1).

Human-driven climate change is causing ongoing long-term changes in the ocean and atmosphere circulation. The effect of added greenhouse gases is to trap additional heat in the Earth system, driving atmospheric and ocean warming (with the latter accounting for more than 90 per cent of the heat trapped so far, Fox-Kemper et al., 2021). There may also be changes in key circulation patterns, with increasing evidence that the Atlantic Meridional Overturning Circulation (AMOC) may be slowing (Dima and Lohmann 2010; Caesar et al., 2018; Rahmstorf et al,. 2015; Zhu et al., 2023). An extra 7 per cent of water vapour can be held by the near-surface atmosphere with every degree of warming, leading to increasing precipitation in some regions (Zika et al, 2018). Evidence shows that heat extremes, heavy rainfall events and agricultural and ecological droughts are already increasing across every continent (IPCC, 2021). As the ocean and atmosphere gradually warm, the range of natural variability around the baseline is shifting upwards, making formerly extreme events more common and formerly impossible events possible. 

Evidence exists from geological records and model simulations that some of these circulation patterns could also feature tipping points, beyond which they may shift to a different state (Lenton et al,. 2008; Armstrong McKay et al. 2022; Wang et al,. 2023). Palaeorecords suggest deep water convection in the North Atlantic has abruptly shifted to a weaker or completely ‘off’ state during previous glacial cycles, with major climatic consequences – a pattern supported by some models (Böhm et al,. 2015; Douville et al., 2021: Fox-Kemper et al., 2021: Ch9 2021). It has also been suggested that the Indian summer monsoon could shift to an alternative state as a result of aerosol emissions, counter to the general trend of monsoon strengthening with warming (Levermann et al. 2009; Doblas-Reyes et al., 2021), and potential shifts in circulations on the southern hemisphere to El Niño-like mean conditions (Fedorov et al, 2006).