hide
Free keywords:
-
Abstract:
The land and ocean absorb on average over half of the anthropogenic emissions of carbon
dioxide (CO2) every year. These CO2 “sinks” are modulated by climate change and
variability. Here we use a suite of nine Dynamic Global Vegetation Models (DGVMs)
5 and four Ocean Biogeochemical General Circulation Models (OBGCMs) to quantify
the global and regional climate and atmospheric CO2 – driven trends in land and
oceanic CO2 exchanges with the atmosphere over the period 1990–2009, attribute
these trends to underlying processes, and quantify the uncertainty and level of model
agreement. The models were forced with reconstructed climate fields and observed
10 global atmospheric CO2; Land Use and Land Cover Changes are not included for the
DGVMs. Over the period 1990–2009, the DGVMs simulate a mean global land carbon
sink of −2.4±0.7 PgCyr−1 with a small significant trend of −0.06±0.03 PgCyr−2
(increasing sink). Over the more limited period 1990–2004, the ocean models simulate
a mean ocean sink of −2.2±0.2 PgCyr−1 with a trend in the net C uptake that
15 is indistinguishable from zero (−0.01±0.02 PgCyr−2). The two ocean models that extended
the simulations until 2009 suggest a slightly stronger, but still small trend of
−0.02±0.01 PgCyr−2. Trends from land and ocean models compare favourably to the
land greenness trends from remote sensing, atmospheric inversion results, and the
residual land sink required to close the global carbon budget. Trends in the land sink
20 are driven by increasing net primary production (NPP) whose statistically significant
trend of 0.22±0.08 PgCyr−2 exceeds a significant trend in heterotrophic respiration of
0.16±0.05 PgCyr−2 – primarily as a consequence of wide-spread CO2 fertilisation of
plant production. Most of the land-based trend in simulated net carbon uptake originates
from natural ecosystems in the tropics (−0.04±0.01 PgCyr−2), with almost no
25 trend over the northern land region, where recent warming and reduced rainfall offsets
the positive impact of elevated atmospheric CO2 on carbon storage. The small uptake
trend in the ocean models emerges because climate variability and change, and in particular increasing sea surface temperatures, tend to counteract the trend in ocean uptake driven by the increase in atmospheric CO2. Large uncertainty remains in the
magnitude and sign of modelled carbon trends in several regions, and on the influence of land use and land cover changes on regional trends.
