Climate change, a major risk for river ecology

In this study coordinated by a geographer of University College London, the risk of reaching an ecological tipping point due to climate change was assessed for 321 watersheds, covering nearly 50% of the world’s surface, with scenarios of increases of global warming of 1, 1.5, 2 and 3° C.

The authors recall that the water regime (variability, magnitude, frequency, duration and discharge rhythm) of a river is crucial for maintaining its aquatic diversity and the integrity of its ecosystem. Healthy aquatic ecosystems are essential for maintaining services provided by rivers to human communities. This study therefore helps the pertinent authorities to foresee certain severe impacts of climate change for human societies.

The study of Earth’s Future is the first to examine the impact of climate change on the ecological functioning of watersheds on the global scale, by assessing the risk of decreases and increases of river discharges. This criterion is a crucial determinant of conditions for aquatic life, in particular the temperature of the water and its oxygen concentration, as well as the connectivity between habitats including flood plains.

Warming synonymous with ecological disruption

The authors made a major discovery: on the global scale, the probability of observing a future characterised by a high risk of ecological disruption due to a modification of river discharges increases with the magnitude of climate change. Their models predict both a considerable lowering and a dramatic increase of discharges and a strong risk of low flows with the highest level of warming, i.e. +3.0° C.

These conclusions, if matched with the inability of governments to act in cooperation to limit the increase in the temperature to below 2.0° C, are especially worrying for the world’s river ecosystems.

Regions of the world are affected unequally

Nonetheless, the study shows that the risks are not distributed equally around the world. The rivers of the boreal regions (which flow northwards in Siberia and North America), for example, are less likely to undergo major ecological changes due to climate change. However, this does not imply that these environments are free of risk, even a low risk! Although this risk is lower than in other regions, it should in no way be ignored: elevated temperatures in these high latitudes will have considerable impacts on ecosystems. The thawing of the permafrost in particular, could lead to an increase of discharges according to the warming scenarios used in the study, and thus represent a major ecological risk.

Climate change and direct human intervention

The study does not take into account human interventions on rivers, which can change their discharges and morphology (reservoirs, extraction of sand from the riverbed, diversion canals, etc.). Conversely, the capacity to regulate rivers can also be used to react to the impacts of climate change on the discharge and temperature of the water!

Historically, the authors state that direct human intervention has not contributed significantly to fluctuations in river discharges on the global scale, in comparison to the indirect effects of climate change of anthropic origin.  Nonetheless, on the scale of the watershed and for several large river basins in Asia (Indus) and the west of the United States (Colorado), human action is responsible for 5 to 15% of the fall in discharges. This means that the study may potentially underestimate the impact of climate change on these basins already subject to heavy pressure due to excessive withdrawals of water. This is also the case for the biodiversity of most of the rivers of Europe, Central Asia, the Middle East, the Indian subcontinent and eastern China. In regions like northern Australia, however, there has been relatively little human intervention on rivers. Therefore, the impact of climate change is more visible.

To reflect on adaptive strategies capable of ensuring the resilience of fluvial ecosystems, the authors emphasise the need to identify the share of contributions linked to climate change and those linked to direct human interventions on rivers.