Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 24

Humankind has had and is still having a direct impact on earth's climate. Since the beginning of the Industrial Age there has been an increase in the concentrations of carbon dioxide (Figure 2.10 top), methane and nitrous oxide in the atmosphere, the direct result of the burning of fossil fuels, and to a lesser extent, deforestation. The increases in atmospheric greenhouse gases have had a direct impact on the global ocean, warming the earth's seas which in turn has resulted in a rise in sea level via thermal expansion. The uptake of CO₂ has resulted in a decrease in ocean pH (Figure 2.10 bottom) and CO₃²⁻ concentration, a process termed ocean acidification. Other impacts of human‐induced climate change include increases in air and ocean temperature, an increase in OHC and changes in precipitation patterns.

Warming of the global ocean is the largest near the surface, the upper 75 m warmed by 0.11 °C per decade over the period 1971–2010 (IPCC 2014). It is likely that the ocean warmed from the 1870s to 1971. Regions of naturally high salinity have become more saline, while regions of low salinity have become fresher since the 1950s. Such regional trends in ocean salinity provide indirect evidence for changes in evaporation and precipitation over the ocean and for changes in the global water cycle (IPCC 2014). Since the beginning of the Industrial Era, oceanic uptake of CO₂ has resulted in a decline in surface ocean pH (Section 2.7.2). There is medium confidence (IPCC 2014) that in parallel to warming, oxygen concentrations have declined in coastal waters and in the open ocean thermocline since the 1960s. OMZs are progressively expanding in the tropical Pacific, Atlantic, and Indian Oceans due to reduced ventilation and oxygen solubility in warmer, more stratified oceans (Stramma et al. 2011).

FIGURE 2.10 Trends in surface (< 50 m depth) ocean carbonate chemistry calculated from observations obtained during the Hawaii Ocean Time‐Series Program in the North Pacific from 1988 to 2017. The upper graph shows the concomitant increase in CO₂ concentrations in both the atmosphere (red) and surface ocean (blue), presented as CO₂ concentration in air (ppm). The bottom graph shows a decline in ocean pH (light blue, primary y‐axis) and carbonate ion (CO₃²⁻) concentration (green, secondary y‐axis on right).

Source: Doney et al. (2020), figure 1, p. 87. Licensed under CC BY 4.0. © Annual Reviews.

Over the period 1901–2010, global mean sea level rose by 0.19 m. The rate of sea‐level rise (SLR) since the mid‐nineteenth century has been larger than the mean rate during the previous two millennia (IPCC 2014). The mean rate of SLR was 1.7 mm a⁻¹ between 1901 and 2010 with an increase to 3.2 mm a⁻¹ between 1993 and 2010. Over this period, global mean SLR has been consistent with the sum of the observed contributions from ocean thermal expansion, the Greenland ice sheet, the Antarctic ice sheet, and land water storage. Rates of mean SLR vary over different regions due to fluctuations in ocean circulation. It is extremely likely (IPCC 2014) that more than half of the observed increase in global average surface temperature and mean SLR from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcing.

The recent climatological forecasts by the Intergovernmental Panel on Climate Change (IPCC) for until the end of this century (Church et al. 2013; Collins et al. 2013; Bindoff et al. 2019; Oppenheimer et al. 2019) predict that globally: (i) SSTs will rise by 1–3 ° C; (ii) oceanic pH will decline by 0.07–0.31 units; and (iii) mean atmospheric CO₂ concentrations will increase to 441 ppm (from 391 ppm in 2011). Regional differences (Table 13.1) will occur for some parameters, such as (i) sea‐level, which will continue to rise globally at an average rate between 1.8–3.6 mm a⁻¹, (ii) precipitation will increase in some regions of the wet tropics and decrease the dry tropics, and (iii) salinity will change in tandem with changes in precipitation.

Rising atmospheric CO₂ and climate change are associated with shifts in temperature, circulation, stratification, nutrient input, oxygen content, and ocean acidification, with potentially wide‐ranging effects on the biology and ecology of marine organisms and their communities (Chapter 13).