The Desert’s Natural Carbon Capacity vs Emission Savings from PV Solar Fields

by Robin Kobaly, Executive Director,The SummerTree Institute

In spite of its rapidly rising popularity, the California desert as an ecosystem remains poorly understood, underfunded, and misperceived. One of the most persistent mischaracterizations is that the California desert is a barren wasteland with low biodiversity and limited capacity for carbon storage. Scientific data refute these inaccuracies, and demonstrate that the California desert has extremely high biodiversity. It is also a significant carbon sink with tremendous capability to sequester carbon and help the state meet its atmospheric carbon reduction goals.

So why isn’t the California desert currently appreciated for its significant role in carbon capture and storage? Mainly because the quantity of carbon storage in the desert is so hard to measure, and because the funding for research to establish that data has been insufficient.

Reasons for the difficulty in calculating the total carbon storage potential in the desert are mostly three-fold:

  1. While above-ground carbon capture and storage can be measured in desert plants, below-ground organic carbon storage is more difficult to measure. This is because carbon-containing desert plant roots and their fungal root partners penetrate soils to depths difficult to access (up to 150 feet deep or more).
  2. Most of the long-term carbon storage in the desert is underground and inorganic, locked into calcite deposits, forming caliche. This is patchy in some places while forming vast layers in others, and is distributed at various depths. This calcite is difficult to locate and measure.
  3. Funding for research to measure both organic and inorganic carbon storage in the desert has been woefully inadequate.

How can we value and appreciate the contribution of desert ecosystems in capturing and storing carbon if the actual amounts of that storage cannot currently be precisely quantified? And then how can the amount of carbon capture lost by removing ancient desert plants on 5,000 acres of intact desert be compared to the carbon prevented from entering the atmosphere by building a 5,000-acre industrial-scale solar field on that same land?

This a dilemma which must be faced by California’s “30-by-30” planning effort (a commitment to conserve 30% of California's land and water by 2030, as codified by AB 1757). A science-focused sub-group of the 30-by-30 Inland Deserts Working Group (IDWG) has been tackling this challenging issue for the past 2-½ years (see the list of team members below*). This team wrote a technical report, "Nature Based Solutions: Desert Sector” (https://desertreport.org), aimed at the California Natural Resources Agency, California Air Resources Board, and California Department of Food and Agriculture. These agencies were urged to commit to preserving intact public lands across the California desert, and to approve the siting of all future large-scale solar projects in the California desert only on land that has already been disturbed, and not on intact desert ecosystems.

Scientists are currently working on ways to measure deeply buried carbon across landscapes like the California Desert that are highly diverse in topography, soils, climate, and vegetation. This is much more challenging than for more homogeneous landscape types. But we do have data that measures how much carbon is accumulated by plants in some desert habitats, and can compare capture rates to other ecosystems around the planet. Carbon capture rates in the desert are truly impressive, even when compared to those of ecosystems commonly considered to be much better carbon sinks.

The primary gauge of an ecosystem’s carbon sink potential is the net exchange of carbon between the ecosystem and the atmosphere, i.e. how much carbon comes in versus how much carbon goes out. This measurement is called “net ecosystem exchange,” or NEE. By comparing the carbon balance of diverse ecosystems, we can get an idea of the relative strength of each ecosystem’s carbon sink capacity.

In our “Desert Sector” report, Dr. Michael Allen (Distinguished Professor Emeritus, Department of Microbiology and Plant Pathology, University of California, Riverside) has summarized NEE measured within various ecosystems world-wide. He has compared them to those measured for two vegetation types found in the California desert: microphyll woodlands (dry washes with small-leaved trees like mesquite, palo verde, and ironwood) and creosote bajada scrub (broad alluvial slopes with creosote bushes).

As shown in the table, the carbon sink capacity of creosote bajada scrub rivals that of a tropical rainforest or boreal forest. Even microphyll woodlands are in the range of coniferous forests in southern California. Just these two of many California desert vegetation types could sequester an average of 1.5 million tons of carbon per year.

Net Ecosystem Exchange Rate

A creosote bajada in the Lucerne Valley.

Photo: Robin Kobaly

c. Pic 1 - creosote bajada

Moreover, these carbon accumulation amounts are not only applicable after rain events in the desert, but can be achieved throughout the year. Habitats with deep-rooted plants, such as microphyll woodlands and creosote bajada scrub, can continue to photosynthesize and capture carbon long after rain events. Because of their long roots that reach to deep, percolated rain water (possibly falling miles away) and even to deeply-buried groundwater, these desert plants can extend their carbon fixation long into drought cycles. The capability for year-round productivity and carbon accumulation within creosote bajadas and microphyll woodlands is extra reason to pursue protection of these habitats against damage and wide-spread destruction from development.

Microphyll Woodland

Photo: Dr. Michael Allen

c. PIc 7 - Microphyll woodland_Mike Allen

Large-scale disturbance of deserts, particularly within critical ecosystems such as creosote bajadas and microphyll woodlands, has the potential to reduce not only California's biodiversity, but also to release inorganic (calcite) carbon stored for millennia. Unlike the organic carbon stored in most ecosystems, much of the desert carbon is stored as inorganic calcites in the soil. Calcite is generated by roots and root-partnering fungi breathing out carbon dioxide underground. This exhaled carbon dioxide combines with calcium to form crystals of calcite, which can eventually form into layers of caliche. If buried and undisturbed, this carbon can remain sequestered for millennia. We estimate that more than 262 million tons of carbon  may be  stored in California deserts as calcites.

c. Pic 5 - Microphyll Plants

Importantly, buried calcites are dissolved upon exposure to air and water. Upon exposure, the carbon dioxide in calcium carbonates can be released from disturbed soils at rates up to 24 kg of carbon per hectare per day following a precipitation event, if the soil is disturbed.

In deserts, the organic carbon of the ecosystem turns over on an average of 38 years, with soil and sediments turning over on a 200-year average. This contrasts with equivalent temperate forest turnovers of 25 and 55 years, cropland turnovers of 22 and 40 years, and perennial grassland turnovers of 36 and 100 years, respectively. Desert organic carbon, once fixed, stays in the system much longer than in other ecosystems, releasing back to the atmosphere slowly.

These slow rates of organic carbon turnover and very long-term inorganic carbon storage in desert ecosystems should be considered when comparing carbon emission savings by a desert solar installation to that of the intact desert ecosystem before disturbance. A true calculation of equivalent carbon emission savings must, if it is honest, subtract carbon no longer sequestered by the destroyed vegetation, as well as carbon being released to the atmosphere by carbon-storing soil now exposed to weathering. It must also account for replacing an independent and self-perpetuating natural ecosystem service with an industrial service requiring continuous maintenance and complete equipment replacement every few decades.

We come back to the issue of comparing carbon capture and storage by an intact desert ecosystem with the carbon emission savings from a comparably-sized photovoltaic solar (PV) project. From existing research, we can get a ballpark idea of the average organic carbon balance of a desert habitat, but we do not have data to accurately quantify the enormous amount of inorganic carbon stored under that habitat. We need to seriously fund research to quantify, measure, and model the desert’s full carbon sequestration capacity.

Our team of scientists is certainly not opposed to PV installations, and we are not even opposed to PV installations in the desert. But we are opposed to siting utility-scale solar fields on intact, undisturbed desert land. There is more than enough disturbed land in California to meet our renewable energy goals, including fallowed agricultural lands, selenium-contaminated lands, parking lots, industrial rooftops, canals, highway corridors, reservoirs, and landfills.

Table-2

Taken from the technical report written by the IDWG for the California 30x30 project,  available at: https://desertreport.org/wp-content/uploads/2023/10/c.-technical-report-on-sequestration.pdf

The potential solar capacity from the options shown above far exceeds the energy agencies’ projected need for an additional 70,000 MW of new utility scale solar to meet the state’s 2045 decarbonization goal. Clearly, the above options are the preferred resources for minimizing land-use impacts and societal costs of developing transmission-dependent solar.

What if we blade thousands of acres of intact desert today because we don’t have the data to show what we are losing? Then, in a few years, when we do get that data, we may realize that we achieved a net carbon loss instead of a net gain. This loss does not even consider the simultaneous losses of biodiversity, damage to wildlife corridors, impacts to human health from dust storms created by soil disruptions, and impacts to ecotourism.

This argument is all about looking at the desert long-term. Nothing in the desert happens quickly. Its processes are slow, and disruptions to the desert ecosystem will take a millennium to repair. We can’t afford to make mistakes in the desert because fixing those mistakes will not be an option.

In summary, does solar in the desert add up? Yes, if it is sited only on previously disturbed land, where native plants have already been removed, soils have been damaged, and the land has already lost its natural capacity for carbon capture and storage. We should work diligently toward a future where all PV arrays in the desert are sited only on previously disturbed lands.

With a Master’s Degree in biology, Robin Kobaly had a twenty-year career as a botanist with the BLM, also serving as a wildlife biologist, and natural history interpreter. She is currently executive director of the SummerTree Institute, a 501(c)3 nonprofit corporation dedicated to providing responsible viewpoints toward our environment, our place in it, and our responsibility to it.

*Team members:

Dr. Michael Allen, Ph.D. Distinguished Professor Emeritus. Department of Microbiology and Plant Pathology, University of California, Riverside

Dr. Cameron Barrows, Ph.D. Conservation Ecologist, Emeritus. Center for Conservation Biology, University of California, Riverside

Colin Barrows, Co-founder, Cactus to Cloud Institute

Susy Boyd, MNR. Master of Natural Resources, Forests and Climate Change, Oregon State University.

Pat Flanagan, B.A. Biology. California State University, Long Beach

Robin Kobaly, M.S. Biology and Plant Ecology, University of California, Riverside

Arch McCulloch, M.S. Computer Science, Azusa Pacific University. B.S.Geology / Computer Science, California State University, Dominguez Hills

Joan Taylor, Governing board of the Coachella Valley Mountains Conservancy, and boards of Friends of the Desert Mountains and The Wildlands Conservancy. Chairperson, California Conservation Committee and California/Nevada Desert Committee of Sierra Club