Publications Published in Nature Geoscience

1. Antarctic temperature and global sea level closely coupled over the past five glacial cycles. E. J. Rohling, K. Grant, M. Bolshaw, A. P. Roberts, M. Siddall, Ch. Hemleben, M. Kucera.
   Nature Geoscience: 2009
   DOI: 10.1038/NGEO557
   Notes
   Ice cores from Antarctica record temperature and atmospheric carbon dioxide variations over the past six glacial cycles1,2. Yet concomitant records of sea-level fluctuations—needed to reveal rates and magnitudes of ice-volume change that provide context to projections for the future3–9—remain elusive. Reconstructions indicate fast rates of sea-level rise up to 5 cm yr
   
   
   
2. Australian climate-carbon cycle feedback reduced by soil black carbon. Lehmann, J.; Skjemstad, J.; Sohi, S.; Carter, J.; Barson, M.; Falloon, P.; Coleman, K.; Woodbury, P.; Krull, E..
   Nature Geoscience: 2008
   DOI: 10.1038/ngeo358
   Notes
   Annual emissions of carbon dioxide from soil organic carbon are an order of magnitude greater than all anthropogenic carbon dioxide emissions taken together(1). Global warming is likely to increase the decomposition of soil organic carbon, and thus the release of carbon dioxide from soils(2-5), creating a positive feedback(6-9). Current models of global climate change that recognize this soil carbon feedback are inaccurate if a larger fraction of soil organic carbon than postulated has a very slow decomposition rate. Here we show that by including realistic stocks of black carbon in prediction models, carbon dioxide emissions are reduced by 18.3 and 24.4% in two Australian savannah regions in response to a warming of 3 degrees C over 100 years(1). This reduction in temperature sensitivity, and thus the magnitude of the positive feedback, results from the long mean residence time of black carbon, which we estimate to be approximately 1,300 and 2,600 years, respectively. The inclusion of black carbon in climate models is likely to require spatially explicit information about its distribution, given that the black carbon content of soils ranged from 0 to 82% of soil organic carbon in a continental-scale analysis of Australia. We conclude that accurate information about the distribution of black carbon in soils is important for projections of future climate change.
   
   
   
3. Climate response to regional radiative forcing during the twentieth century. Drew Shindell, Greg Faluvegi.
   Nature Geoscience: 2009
   DOI: 10.1038/NGEO473
   Notes
   Regional climate change can arise from three different effects: regional changes to the amount of radiative heating that reaches the Earth’s surface, an inhomogeneous response to globally uniform changes in radiative heating and variability without a specific forcing. The relative importance of these effects is not clear, particularly because neither the response to regional forcings nor the regional forcings themselves are well known for the twentieth century. Here we investigate the sensitivity of regional climate to changes in carbon dioxide, black carbon aerosols, sulphate aerosols and ozone in the tropics, mid-latitudes and polar regions, using a coupled ocean–atmosphere model. We find that mid- and high-latitude climate is quite sensitive to the location of the forcing. Using these relationships between forcing and response along with observations of twentieth century climate change, we reconstruct radiative forcing from aerosols in space and time. Our reconstructions broadly agree with historical emis ions estimates, and can explain the differences between observed changes in Arctic temperatures and expectations from non-aerosol forcings plus unforced variability. We conclude that decreasing concentrations of sulphate aerosols and increasing concentrations of black carbon have substantially contributed to rapid Arctic warming during the past three decades.
   
   
   
4. Climate sensitivity to the carbon cycle modulated by past and future changes in ocean chemistry. Goodwin, P.; Williams, R. G.; Ridgwell, A.; Follows, M. J..
   Nature Geoscience: 2009
   DOI: 10.1038/ngeo416
   Notes
   The carbon cycle has a central role in climate change. For example, during glacial-interglacial cycles, atmospheric carbon dioxide has altered radiative forcing and amplified temperature changes. However, it is unclear how sensitive the climate system has been to changes in carbon cycling in previous geological periods, or how this sensitivity may evolve in the future, following massive anthropogenic emissions. Here we develop an analytical relationship that links the variation of radiative forcing from changes in carbon dioxide concentrations with changes in air-sea carbon cycling on a millennial timescale. We find that this relationship is affected by the ocean storage of carbon and its chemical partitioning in sea water. Our analysis reveals that the radiative forcing of climate is more sensitive to carbon perturbations now than it has been over much of the preceding 400 million years. This high sensitivity is likely to persist into the future as the oceans become more acidic and the bulk of the fossil-fuels inventory is transferred to the ocean and atmosphere.
   
   
   
5. Committed terrestrial ecosystem changes due to climate change. Chris Jones, Jason Lowe, Spencer Liddicoat, Richard Betts.
   Nature Geoscience: 2009
   DOI: 10.1038/NGEO555
   Notes
   Targets for stabilizing climate change are often based on considerations of the impacts of different levels of global warming, usually assessing the time of reaching a particular level of warming. However, some aspects of the Earth system, such as global mean temperatures1 and sea level rise due to thermal expansion2 or the melting of large ice sheets3, continue to respond long after the stabilization of radiative forcing. Here we use a coupled climate–vegetation model to show that in turn the terrestrial biosphere shows significant inertia in its response to climate change. We demonstrate that the global terrestrial biosphere can continue to change for decades after climate stabilization. We suggest that ecosystems can be committed to long-term change long before any response is observable: for example, we find that the risk of significant loss of forest cover in Amazonia rises rapidly for a global mean temperature rise above 2 C. We conclude that such committed ecosystem changes must be considered in the efinition of dangerous climate change, and subsequent policy development to avoid it.
   
   
   
6. Global ammonia distribution derived from infrared satellite observations. Lieven Clarisse, Cathy Clerbaux, Frank Dentener, Daniel Hurtmans, Pierre-François Coheur.
   Nature Geoscience: 2009
   DOI: 10.1038/NGEO551
   Notes
   Global ammonia emissions have more than doubled since pre-industrial times, largely owing to agricultural intensification and widespread fertilizer use1. In the atmosphere, ammonia accelerates particulate matter formation, thereby reducing air quality. When deposited in nitrogen-limited ecosystems,ammonia can act as a fertilizer. This can lead to biodiversity reductions in terrestrial ecosystems, and algal blooms in aqueous environments2–8. Despite its ecological significance, there are large uncertainties in the magnitude of ammonia emissions, mainly owing to a paucity of ground-based observations and a virtual absence of atmospheric measurements3,8–11. Here we use infrared spectra, obtained by the IASI/MetOp satellite, to map global ammonia concentrations from space over the course of 2008. We identify several ammonia hotspots in middle–low latitudes across the globe. In general, we find a good qualitative agreement between our satellite measurements and simulations made using a global atmospheric chemistry transport model. However, the satellite data reveal substantially higher concentrations of ammonia north of 30 N, compared with model projections. We conclude that ammonia emissions could have been significantly underestimated in the Northern Hemisphere, and suggest that satellite monitoring of ammonia from space will improve our understanding of the global nitrogen cycle.
   
   
   
7. Global and regional climate changes due to black carbon. Ramanathan, V.; Carmichael, G..
   Nature Publishing Group Nature Geoscience: 2008
   Notes
   Black carbon in soot is the dominant absorber of visible solar radiation in the atmosphere. Anthropogenic sources of black carbon, although distributed globally, are most concentrated in the tropics where solar irradiance is highest. Black carbon is often transported over long distances, mixing with other aerosols along the way. The aerosol mix can form transcontinental plumes of atmospheric brown clouds, with vertical extents of 3 to 5 km. Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions. In the Himalayan region, solar heating from black carbon at high elevations may be just as important as carbon dioxide in the melting of snowpacks and glaciers. The interception of solar radiation by atmospheric brown clouds leads to dimming at the Earth's surface with important implications for the hydrological cycle, and the deposition of black carbon darkens snow and ice surfaces, which can contribute to melting, in particular of Arctic sea ice.
   
   
   
8. High rates of sea-level rise during the last interglacial period. Rohling, E. J.; Grant, K.; Hemleben, Ch; Siddall, M.; Hoogakker, B. A. A.; Bolshaw, M.; Kucera, M..
   Nature Publishing Group Nature Geoscience: 2008
   
   
9. High sensitivity of peat decomposition to climate change through water-table feedback. Ise, T.; Dunn, A. L.; Wofsy, S. C.; Moorcroft, P. R..
   Nature Geoscience: 2008
   DOI: 10.1038/ngeo331
   Notes
   Historically, northern peatlands have functioned as a carbon sink, sequestering large amounts of soil organic carbon, mainly due to low decomposition in cold, largely waterlogged soils(1,2). The water table, an essential determinant of soil-organic-carbon dynamics(3-10), interacts with soil organic carbon. Because of the high water-holding capacity of peat and its low hydraulic conductivity, accumulation of soil organic carbon raises the water table, which lowers decomposition rates of soil organic carbon in a positive feedback loop. This two-way interaction between hydrology and biogeochemistry has been noted(3,5-8), but is not reproduced in process-based simulations(9). Here we present simulations with a coupled physical-biogeochemical soil model with peat depths that are continuously updated from the dynamic balance of soil organic carbon. Our model reproduces dynamics of shallow and deep peatlands in northern Manitoba, Canada, on both short and longer timescales. We find that the feedback between the water table and peat depth increases the sensitivity of peat decomposition to temperature, and intensifies the loss of soil organic carbon in a changing climate. In our long-term simulation, an experimental warming of 4 degrees C causes a 40% loss of soil organic carbon from the shallow peat and 86% from the deep peat. We conclude that peatlands will quickly respond to the expected warming in this century by losing labile soil organic carbon during dry periods.
   
   
   
10. Identifying the causes of sea-level change. Glenn A. Milne, W. Roland Gehrels, Chris W. Hughes and Mark E. Tamisiea.
    Nature Geoscience:
    DOI: 10.1038/ngeo544
    Notes
    Global mean sea-level change has increased from a few centimetres per century over recent millennia to a few tens of centimetres per century in recent decades. This tenfold increase in the rate of rise can be attributed to climate change through the melting of land ice and the thermal expansion of ocean water. As the present warming trend is expected to continue, global mean sea level will continue to rise. Here we review recent insights into past sea-level changes on decadal to millennial timescales and how they may help constrain future changes. We find that most studies constrain global mean sea-level rise to less than one metre over the twenty-first century, but departures from this global mean could reach several decimetres in many areas. We conclude that improving estimates of the spatial variability in future sea-level change is an important research target in coming years.