The Devastating Impact of Anthropogenic Climate Change on the Colorado River Basin

The Colorado River Basin has long been a vital source of freshwater for over 40 million people in the arid southwestern United States. Unfortunately, the past two decades have witnessed a devastating megadrought, lasting multiple decades, that has severely depleted the basin’s water resources. This megadrought, which occurred between 2000 and 2021, is believed to be directly linked to anthropogenic climate change. Researchers, led by Dr. Benjamin Bass and his team at the University of California, have conducted a comprehensive study to shed light on the extent of the impact.

Unprecedented Dry Spells and Declining River Flow

During the years 2020 and 2021, the Colorado River Basin experienced the driest 20-month period since 1895. The river flow also reached its lowest point since 1906. These alarming statistics serve as a wake-up call to the severity of the situation. Dr. Bass and his colleagues sought to examine the changes in precipitation and runoff within the basin since the 1880s, taking into account the 1.5°C increase in temperature over the same period.

Their research, recently published in Water Resources Research, reveals a staggering 10.3% decrease in runoff within the basin. This decline is a direct consequence of both anthropogenic warming and changes in vegetation across the landscape. Consequently, the available water resources to support the local population have dwindled by a concerning 2.1 km3.

The Vulnerability of Snowpack Regions

One significant finding of the study is the disproportionate impact of aridification on the snowpack regions within the Colorado River Basin. Although these regions represent merely 30% of the total area, they have suffered a staggering 86% decrease in runoff. For every degree Celsius of warming, these regions lose an alarming 1.2 km3 of water.

This exacerbation in the decline of runoff in snowpack regions is attributed to albedo feedback. As the snowpack diminishes, its lighter surface, which normally reflects solar heat, is progressively replaced by darker land surfaces that absorb heat. This process further increases the temperature, leading to more snowmelt. Thus, a destructive feedback loop is established, perpetuating the loss of water resources.

Anthropogenic Changes and Alarming Projections

By utilizing Global Climate Models and historical data, the researchers conducted simulations to assess the impact of anthropogenic changes on runoff trends. They also predicted scenarios in which the influence of human activities is eliminated. Their findings indicate that since 1954, there has been a 1.2 km3 decrease in drainage basin runoff. However, they estimate that without the influence of global warming and elevated CO2, runoff would have actually increased by 0.9 km3.

Over the years, atmospheric carbon dioxide levels have risen significantly. In 1880, CO2 levels were estimated to be 285 ppm (parts per million). By 1950, they had risen to 313 ppm, and in 2021, they reached a staggering 416 ppm. The correlation between increasing CO2 levels, climate change, and declining runoff is evident. The pace of this decline has notably accelerated since the 1980s.

Vegetation plays a crucial role in the drainage of the river basin by counterbalancing runoff. The researchers highlight the impact of elevated CO2 levels on vegetation. As CO2 levels rise, the stomata on leaves close, reducing transpiration (the release of water vapor). However, elevated temperatures lead to increased evaporation from the leaf surface, outweighing any water resource efficiency gained from reduced transpiration. This ultimately results in a net loss of water from plants. Although vegetation does contribute to offsetting runoff losses by 15%, its effectiveness depends on the type and coverage of vegetation within the basin.

The megadrought experienced since 2000 has witnessed a 0.48°C temperature increase and a 3.1% reduction in precipitation compared to the climate average. In late 2021 to early 2022, this dire situation worsened, with temperatures soaring 0.83°C above the climate mean. Additionally, precipitation and runoff decreased by 23.9% and 38.6% respectively during this period.

The cumulative effect of this megadrought has resulted in a total decline in runoff of 40.1 km3. In the more recent 2020 to 2021 event, the decline reached 3 km3. The repercussions of these water shortages have led to Arizona, Nevada, and Mexico being declared under a water shortage in 2021. Their freshwater allocation was subsequently reduced by 0.756 km3, a volume greater than the capacity of Nevada’s Lake Mead, the largest reservoir in the United States.

Given that the mean annual runoff for the Colorado River Basin is 21.2 km3, the loss of approximately 10% of its volume over the past 140 years is of significant concern. As CO2 levels and temperatures are projected to continue rising, the rate of water loss will only escalate further. This poses severe consequences for the millions of people who rely on the Colorado River Basin’s water resources every day.

The research conducted by Dr. Benjamin Bass and his team serves as a stark reminder of the devastating impact of anthropogenic climate change on the Colorado River Basin. Urgent action is needed to mitigate the effects of climate change and ensure the long-term sustainability of this crucial water source.