Irreversible Land Water Availability Changes From a Potential ITCZ Shift During Temperature Overshoot

• Published in: Earth's future Vol. 13; no. 5
• Main Authors: Steinert, Norman Julius, Schwinger, Jörg, Chadwick, Robin, Kug, Jong‐Seong, Lee, Hanna
• Format: Journal Article
• Language: English
• Published: Bognor Regis John Wiley & Sons, Inc 01.05.2025; Wiley
• Subjects: Availability; Carbon dioxide; Carbon dioxide removal; Climate change; Climate models; Emissions; Emissions control; Evaporation; Global temperatures; Hydrologic cycle; Hydrologic regime; hydrological cycle; Intertropical convergence zone; precipitation minus evaporation (P − E); Seasonal variations; temperature overshoot; Tropical environment; Tropical environments; Water availability; World population
• ISSN: 2328-4277; 2328-4277
• DOI: 10.1029/2024EF005787

Without rapid emission reduction, it is increasingly likely that global temperatures will overshoot 1.5°C before carbon dioxide removal may help reverse warming. Such temperature overshoots affect the future hydrological cycle, with implications for land water availability. However, the hydrological response to such temperature overshoots is not well understood. Here, we investigate regional and seasonal changes of precipitation minus evaporation (P − E) in an ensemble of Earth system model simulations of temperature overshoot. Most climate models broadly show P − E reversibility after overshoot. However, models exhibiting an irreversible shift of the intertropical convergence zone (ITCZ) during the temperature overshoot experience reduced wet‐season and enhanced dry‐season land water availability in tropical regions, which has long‐lasting effects on the amplitude of P − E seasonality after the overshoot, constituting irreversible changes on human timescales. While some regions may experience alleviating seasonal hydrological conditions, others are subject to more intense seasonality. Half a century of CO2‐stabilization after the temperature overshoot only halves the legacy effects of the overshoot on land water availability on over 23% of the world population in 12% of the global land area, covering regions of various hydrological regimes. Based on the model ensemble presented here, a strong irreversible shift of the ITCZ after an overshoot is a low‐probability but high‐impact outcome that would entail long‐lasting hydrological changes with consequences for ecosystems and human societies. Plain Language Summary This study examines how a temporary overshoot in global temperatures could permanently change regional and seasonal water availability. Using climate models, we explored how the global hydrological cycle, measured as the balance of rainfall and evaporation, might respond to such a scenario. Most models predict that water availability would eventually return to pre‐overshoot levels, but a few suggest otherwise. A significant finding involves the intertropical convergence zone (ITCZ), a band of rainfall near the equator. In certain scenarios, the ITCZ shifts southward during the temperature overshoot and does not fully return to its original position afterward. This leads to lasting changes in the wet and dry seasons, particularly in tropical areas such as parts of Africa, the Amazon, and Southeast Asia. These changes could impact over 1.8 billion people by altering regional water availability patterns, for example, increasing water scarcity in some areas while causing excessive rainfall in others. Hence, our research highlights the risk of low‐probability but high‐impact outcomes from overshoot scenarios. These long‐term changes could disrupt ecosystems, agriculture, and water supplies, particularly in vulnerable regions. Key Points A strong intertropical convergence zone (ITCZ) shift occurs in 1–2 of 8 CMIP6 models simulating overshoot, not depicting the most likely but a plausible high‐impact outcome The ITCZ shift drives irreversible hydrological changes, affecting wet and dry season dynamics long after a return to pre‐industrial levels More than 1.8 billion people may face altered water availability due to overshoot impacts, especially in vulnerable tropical regions