ABSTRACT

The photic zones of the oceans—where sunlight and moonlight drive ecological interactions—are one of the most productive habitats on the planet and fundamental to the maintenance of healthy global biogeochemical cycles. Ocean darkening occurs when changes in the optical properties of the oceans reduce the depth to which sufficient light penetrates to facilitate biological processes guided by sunlight and moonlight. We analysed a 9 km resolution annual time series of MODIS Aqua's diffuse attenuation coefficient of light at 490 nm [Kd(490)] to quantify whether the oceans have darkened over the last 20 years and the impact of this on the depth of photic zones around the world. Kd(490) increased across 75,341,181 km2 (21%) of the global ocean between 2003 and 2022, resulting in photic zone depths reducing by more than 50 m across 32,449,129 km2 (9%) by area. The depth of the photic zone has reduced by more than 10% across 32,446,942 km2 (9%) of the global ocean. Our analysis indicates that ocean darkening is not restricted to coastal regions, but affects large swathes of the open ocean. A combination of nutrient, organic material and sediment loading near the coasts and changes in global ocean circulation are probable causes of increases in primary and secondary productivity that have reduced light penetration into surface waters. The implications of ocean darkening for marine ecology and the ecosystem services provided by the surface oceans are currently unknown but likely to be severe.

Abstract

Drought is one of the most common and complex natural hazards affecting the environment, economies and populations globally1,2,3,4. However, there are significant uncertainties in global drought trends4,5,6, and a limited understanding of the extent to which a key driver, atmospheric evaporative demand (AED), impacts the recent evolution of the magnitude, frequency, duration and areal extent of droughts. Here, by developing an ensemble of high-resolution global drought datasets for 1901–2022, we find an increasing trend in drought severity worldwide. Our findings suggest that AED has increased drought severity by an average of 40% globally. Not only are typically dry regions becoming drier but also wet areas are experiencing drying trends. During the past 5 years (2018–2022), the areas in drought have expanded by 74% on average compared with 1981–2017, with AED contributing to 58% of this increase. The year 2022 was record-breaking, with 30% of the global land area affected by moderate and extreme droughts, 42% of which was attributed to increased AED. Our findings indicate that AED has an increasingly important role in driving severe droughts and that this tendency will likely continue under future warming scenarios.

Abstract

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.