PNNL

Abstract

Water temperature is a key abiotic factor influencing aquatic ecosystem health and the services they provide for both nature and humans. Global water temperature models offer possibilities to improve our understanding of water temperature regimes, which is increasingly important against the backdrop of climate change. Yet, most previous studies have predominantly relied on a single model, which can lead to an incomplete representation of uncertainty, potential biases, and limited insight into the range of possible future conditions, which ultimately reduces the robustness of climate impact assessments. Here, we provide a comprehensive assessment of surface freshwater temperature changes from various river and lake models for both past conditions and under future scenarios of climate change. Global models consistently simulate that water temperatures are now 0.5-0.8 ºC higher than at the turn of the century, and that warming will extend and intensify with future global change throughout the 21st century. While the strength of warming is highly sensitive to the choice of water temperature model, emissions scenario and global climate model, our multi-model ensemble shows a global average annual water temperature rise of between +1.3 ºC and +4.1 ºC by the end of the century. Water temperature anomalies are also projected to get progressively more seasonal towards the end-of-the-century – especially under the more extreme climate change scenarios and during summer months in the Northern Hemisphere – with severe implications for the viability of existing thermoelectric power plants. Given the challenges associated with (large-scale) adaptation to control water temperature regimes, strong climate change mitigation is crucial for minimising water temperature rises and its associated negative impacts on humankind and ecosystems.