finding 15.2 : partial-offset-warming-effect-of-carbon-storage

In total, land in the United States absorbs and stores an amount of carbon equivalent to about 17% of annual U.S. fossil fuel emissions. U.S. forests and associated wood products account for most of this land sink. The effect of this carbon storage is to partially offset warming from emissions of CO₂ and other greenhouse gases.

This finding is from chapter 15 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: High confidence. Evidence for human inputs of C, N, and P come from academic, government, and industry sources. The data show substantial agreement. The likelihood of continued dominance of CO2 over other greenhouse gases as a driver of global climate change is also judged to be high, because its concentration is an order of magnitude higher and its rate of change is well known.

Description of evidence base: The author team evaluated technical input reports (17) on biogeochemical cycles, including the two primary sources.4b55a6d6-94ab-4ca9-abaf-e17bf9d86bc4 6b1b7945-4773-4923-8a45-3dc034dff5f8 The “Estimating the U.S. Carbon Sink” section relies on multiple sources of data that are described therein. Numerous studies of the North American and U.S. carbon sink have been published in reports and the scientific literature. Estimates of the percentage of fossil fuel CO2 emissions that are captured by forest, cropland, and other lands vary from a low of 7% to a high of about 24%, when the carbon storage is estimated from carbon inventories.b130f588-b962-4133-9aa9-46f1f8c8ffdb 3f886db6-ca75-43db-988a-3727ebc687f4 d1fa458e-26e1-489b-82b7-1e12b02a730e The forest sink has persisted in the U.S. as forests that were previously cut have regrown. Further studies show that carbon uptake can be increased to some extent by a fertilization effect with reactive nitrogenc744676f-c7f4-404e-96f9-cfc0dbbf8445 248c9e2d-3461-43df-93a3-4f34d39ad99c and phosphorus,2caad53b-a039-47bb-a1fc-391522eb0949 eaa34c7b-6f78-41cb-8dcd-b1abece4cfb0 07bbfceb-3995-48c9-9483-e8f0d9e11dbb both nutrients that can limit the rate of photosynthesis. The carbon sink due to nitrogen fertilization is projected to lessen in the future as controls on nitrogen emissions come into play.061d6ff2-42ec-4051-8c3f-416e30680df0 While carbon uptake by ecosystems has a net cooling effect, trace gases emitted by ecosystems have a warming effect that can offset the cooling effect of the carbon sink.f2b357c2-f4ae-4868-a058-e48fbdbb1303 The most important of these gases are methane and nitrous oxide (N2O), the concentrations of which are projected to rise.9b454e35-76b7-49d3-ac39-333496dbe6a1 f2b357c2-f4ae-4868-a058-e48fbdbb1303 9eb0ce14-5233-46ce-ab54-73ae5575f658 5e813380-eaa6-4f7d-920f-976378f8b163 e7a95f56-ba30-4584-8565-fe22602dcd6f 642f3738-fa2f-44fe-a9c6-c0b33a24b87c

New information and remaining uncertainties: The carbon sink estimates have very wide margins of error. The percent of U.S. CO2 emissions that are stored in ecosystems depends on which years are used for emissions and whether inventories, ecosystem process models, atmospheric inverse models, or some combination of these techniques are used to estimate the sink size (see “Estimating the U.S. Carbon Sink”). The inventories are continually updated (for example, EPA 201313e4d075-f7bd-4fd4-a41f-a1f0cd93356d), but there is a lack of congruence on which of the three techniques is most reliable. A recent paper that uses atmospheric inverse modeling suggests that the global land and ocean carbon sinks are stable or increasing.3da70ec7-f7bd-4575-959c-d39ef1881dca While known to be significant, continental-scale fluxes and sources of the greenhouse gases N2O and CH4 are based on limited data and are potentially subject to revision. Recent syntheses061d6ff2-42ec-4051-8c3f-416e30680df0 evaluate the dynamics of these two important gases and project future changes. Uncertainties remain high.

Assessment of confidence based on evidence: We have very high confidence that the value of the forest carbon sink lies within the range given, 7% to 24% (with a best estimate of 16%) of annual U.S. greenhouse gas emissions. There is wide acceptance that forests and soils store carbon in North America, and that they will continue to do so into the near future. The exact value of the sink strength is very poorly constrained, however, and knowledge of the projected future sink is low. As forests age, their capacity to store carbon in living biomass will necessarily decrease,b6334b6e-1284-4157-871d-8c3c5b5e594f but if other, unknown sinks are dominant, ecosystems may continue to be a carbon sink. We have high confidence that the combination of ecosystem carbon storage of human-caused greenhouse gas emissions and potential warming from other trace gases emitted by ecosystems will ultimately result in a net warming effect. This is based primarily on one recent synthesis,061d6ff2-42ec-4051-8c3f-416e30680df0 which provides ranges for multiple factors and describes the effects of propagating uncertainties. However, the exact amount of warming or cooling produced by various gases is not yet well known, because of the interactions of multiple factors.

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