Climate Change and it's Effect on the Salinity of San Francisco Bay Estuary

The amount of carbon dioxide in the atmosphere has increased by 25% since the Industrial Revolution. This is due in part to anthropogenic actions such as combustion of fossil fuels and widespread deforestation. Carbon dioxide and other greenhouse gases affect the Earth's climate because they are radiatively active. This means that they absorb thermal radiation emitted from the Earth’s surface and reradiate it back to Earth rather than harmlessly letting that 'heat' leave Earth altogether (9). Environmental scientists cannot agree on the magnitude of climate change predicted to affect the Earth in the near future resulting from these greenhouse gases. Estimates from various models range from 1.4-5.8 °C over the next 100 years (3). Despite this, there is a consensus that the Earth's surface is warming at an unnaturally accelerated rate and will continue to do so (9).

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The strength, timing, and volume of runoff of freshwater to coastal ecosystems is controlled by the hydrologic cycle. A change in the Earth's climate would most likely affect that cycle (10). This would clearly affect all estuaries, but because the San Francisco Bay Estuary relies so heavily on artificial (reservoirs) and natural (snowpack) freshwater storage, it is particularly susceptible to the potential effects of global warming (3,4). The general result of a warmer climate and the resulting effects on precipitation and the amount of snow would be to raise the salinity in the San Francisco Estuary (3).  This salinity increase would alter stratification and mixing of fresh and saline water, thereby reshaping species distributions, life histories, and biogeochemistry (10).

The most severe potential impact of climate warming in arid and semiarid regions such as the San Francisco Bay area is the loss of alpine snow volume. Snowmelt is a vital dry season water source and as mentioned in the background, this area receives at least 40% of its freshwater from snowpack runoff. In the past years, observations of warmer winter temperatures, less snow cover, and earlier spring seasons have intensified unease concerning the effect of global warming on alpine snowpack (11).

In addition to California's already lacking supply of water resources, regional climate models estimate up to a 60% reduction in snowpack over the next 100 years. This is due to a climate warming of up to 3 °C in response to a buildup of greenhouse gases in the atmosphere (11, 3, 4, ). Warmer temperatures increase the height of the freezing line, thus causing less snow accumulation and increased melting below this point (12). It is worth noting that the hydrologic simulations based on climate model projections and specified incremental temperature and precipitation changes that bracket the range of possible outcomes are not perfect. Interpreting the results should be somewhat qualitative and focus more on trends than numbers, due to the overall uncertainty in model projections (12).

The models predict the loss of snowpack from the imposed climate warming results in higher runoff peaks before April and less snowmelt-driven flows in the dry season. Over 3 km3 are proposed to shift from flowing during the dry season, to just before it. Although total freshwater inflow is conserved annually, the lack of inflows during the spring and summer have a greater effect than do the winter increases. This is due to residence times in the estuary. The spring and summer season has a longer residence time than does the winter season, meaning that the increased freshwater is flushed out of the estuary before April. After that, the estuary does not receive the normal snowmelt inflow, so the lost freshwater is replaced by seawater. This translates into a higher salinity in the estuary during the dry season, which accumulates gradually every year. Average salinity during the summer months is predicted to increase by 2.2 practical salinity units (psu) (3).

Increased and earlier freshwater inflow will significantly alter salinity regimes. (10,12).  If this effect strengthens significantly over the next century, the increase of winter freshwater inputs and the decrease of spring and summer inflow will decrease salinities in the wet season and, by a larger degree, increase salinity levels in the dry season, further stressing a fragile ecosystem (10).