The changing carbon cycle at Mauna Loa Observatory

Abstract

The amplitude of the CO2 seasonal cycle at the Mauna Loa Observatory (MLO) increased from the early 1970s to the early 1990s but decreased thereafter despite continued warming over northern continents. Because of its location relative to the large-scale atmospheric circulation, the MLO receives mainly Eurasian air masses in the northern hemisphere ( atmospheric circulation ͉ atmospheric CO2 seasonal cycle ͉ terrestrial carbon sinks ͉ continental droughts T he time series of CO 2 at Mauna Loa Observatory (MLO) located on the Island of Hawaii is unique not only because of its accuracy and length but also because it was designed and has been repeatedly demonstrated to capture the globally averaged secular trend in atmospheric CO 2 . The seasonal cycle of atmospheric CO 2 at the MLO, with a maximum at the beginning of the growing season (May) and a minimum at the end of the growing season (September/October), records the ''breathing'' of the northern hemisphere (NH) biosphere, that is, the seasonal asynchrony between photosynthetic drawdown and respiratory release of CO 2 by terrestrial ecosystems (e.g., refs. 1-3).During the course of a year, the MLO experiences marked shifts in large-scale atmospheric circulation. In the NH cold season, air masses from Eurasia dominate transport to the MLO as a result of a deepening of the Aleutian Low and intensified midlatitude westerly flow (4). In the NH warm season, the dominant subtropical North Pacific high-pressure system located to the northeast of the MLO leads to short-range transport of air masses to the MLO that originate over or near the North American continent (4). During this season, the MLO receives an approximately equal mix of air masses from both continents with relatively more North American contributions at the peak of the NH growing season (July/August). The MLO is also located in the vicinity of the subsiding branch of the Hadley circulation, rendering it sensitive to interhemispheric mixing.Several studies have analyzed variability in the MLO CO 2 seasonal cycle to infer the sensitivity of ecosystem dynamics to climate perturbations. The increasing trends in the seasonal amplitude of CO 2 at the MLO and Point Barrow, AK, from the early 1970s to the early 1990s are postulated to be evidence of a temperature-related lengthening of the boreal growing season (1

Elevation-dependent influence of snow accumulation on forest greening

“…The extent to which snow accumulation can explain variations in greenness varies with elevation, reaching a maximum in the water-limited midelevations, between 2,000 and 2,600 m. In situ measurements of carbon uptake and snow accumulation along an elevational transect in the region confirm the elevation dependence of this relationship. We suggest that mid-elevation mountain forest ecosystems could prove particularly sensitive to future increases in temperature and concurrent changes in snow accumulation and melt.Recent studies have documented a shift from energy to water limitation across forested ecosystems of western North America 5,6 . This transformation has reversed the response of these ecosystems to increases in temperature where before the early 1990s increases in air temperature increased terrestrial carbon uptake.…”

“…Recent studies have documented a shift from energy to water limitation across forested ecosystems of western North America 5,6 . This transformation has reversed the response of these ecosystems to increases in temperature where before the early 1990s increases in air temperature increased terrestrial carbon uptake.…”

Influence of Droughts on Mid-Tropospheric CO2

“…In addition to the trend [1][2][3] and the annual cycle [4][5][6][7][8][9], atmospheric CO 2 also demonstrates a lot of intra-seasonal [10,11] and inter-annual variability [12][13][14][15]. During the pre-satellite era, CO 2 analyses mainly utilized in-situ CO 2 measurements to investigate CO 2 variability, which is limited in space.…”

“…Large GPP means more photosynthesis, thus more CO 2 uptake from biosphere and less CO 2 in the atmosphere. Using CO 2 data over Mauna Loa, Buermann et al [8] found that the decline in the CO 2 seasonal cycle amplitudes since the early 1990s is related to reductions in the carbon sequestration as responses to severe droughts and changes in the atmospheric circulation. Norman et al [24] measured soil surface CO 2 fluxes at three sites and found that the soil surface CO 2 fluxes are sensitive to the drought and temperature.…”

“…Parton et al [28] analyzed net ecosystem production on the short grass steppe vegetation and found that net carbon uptake is reduced during low precipitation events. These pervious analyses [8,[24][25][26][27][28] mainly focus on measurements over a small area. Global distributed CO 2 data from AIRS offer a unique opportunity to explore the impact of droughts on atmospheric CO 2 over large spatial domain.…”