Christian H. Reick

Climate-carbon cycle feedback analysis: Results from the C4MIP model intercomparison

Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850–2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0.1° and 1.5°C. All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models.

Twenty-First-Century Compatible CO2 Emissions and Airborne Fraction Simulated by CMIP5 Earth System Models under Four Representative Concentration Pathways

AbstractThe carbon cycle is a crucial Earth system component affecting climate and atmospheric composition. The response of natural carbon uptake to CO2 and climate change will determine anthropogenic emissions compatible with a target CO2 pathway. For phase 5 of the Coupled Model Intercomparison Project (CMIP5), four future representative concentration pathways (RCPs) have been generated by integrated assessment models (IAMs) and used as scenarios by state-of-the-art climate models, enabling quantification of compatible carbon emissions for the four scenarios by complex, process-based models. Here, the authors present results from 15 such Earth system GCMs for future changes in land and ocean carbon storage and the implications for anthropogenic emissions. The results are consistent with the underlying scenarios but show substantial model spread. Uncertainty in land carbon uptake due to differences among models is comparable with the spread across scenarios. Model estimates of historical fossil-fuel emis...

What do moisture recycling estimates tell? Lessons from an extreme global land-cover change model experiment

Moisture recycling estimates are diagnostic measures that could ideally be used to deduce the response of precipitation to modified land-evaporation. Recycling estimates are based on moisture-budget considerations in which water is treated as a passive tracer. But in reality water is a thermodynamically active component of the atmosphere. Accordingly, recycling estimates are applicable to deduce the response to a perturbation only if other mechanisms by which evaporation affects climate do not dominate the response – a condition that has not received sufficient attention in the literature. In our analysis of what moisture recycling estimates tell us, we discuss two such additional mechanisms that result from water’s active role. These are (I) local coupling, by which precipitation is affected locally via the thermal structure of the atmosphere, and (II) the atmospheric circulation, by which precipitation is affected on a large spatial scale. We perform two global climate model experiments: One with and another without continental evaporation. By this extreme perturbation we test the predictive utility of a certain type of recycling measure, the “continental recycling ratio”. Moreover, by such a strong perturbation the whole spectrum of possible responses shows up simultaneously, giving us the opportunity to discuss all concurrent mechanisms jointly. The response to this extreme perturbation largely disagrees with the hypothesis that moisture recycling is the dominant mechanism. Instead, most of the response can be attributed to changes in the atmospheric circulation, while the contributions to the response by moisture recycling as well as local coupling, though noticeable, are smaller. By our case study it is not possible to give a general answer to the question posed Correspondence to: H. F. Goessling (helge.goessling@zmaw.de) in the title, but it demonstrates that recycling estimates do not necessarily mirror the consequences of land-use change for precipitation.

Sensitivity of a coupled climate‐carbon cycle model to large volcanic eruptions during the last millennium

The sensitivity of the climate–biogeochemistry system to volcanic eruptions is investigated using the comprehensive Earth System Model developed at the Max Planck Institute for Meteorology. The model includes an interactive carbon cycle with modules for terrestrial biosphere as well as ocean biogeochemistry. The volcanic forcing is based on a recent reconstruction for the last 1200 yr. An ensemble of five simulations is performed and the averaged response of the system is analysed in particular for the largest eruption of the last millennium in the year 1258. After this eruption, the global annual mean temperature drops by 1 K and recovers slowly during 10 yr. Atmospheric CO2 concentration declines during 4 yr after the eruption by ca. 2 ppmv to its minimum value and then starts to increase towards the pre-eruption level. This CO2 decrease is explained mainly by reduced heterotrophic respiration on land in response to the surface cooling, which leads to increased carbon storage in soils, mostly in tropical and subtropical regions. The ocean acts as a weak carbon sink, which is primarily due to temperature-induced solubility. This sink saturates 2 yr after the eruption, earlier than the land uptake.

Contribution of anthropogenic land cover change emissions to pre-industrial atmospheric CO2.

Based on a recent reconstruction of anthropogenic land cover change (ALCC), we derive the associated CO2 emissions since 800ADby two independent methods: a bookkeeping approach and a process model. The results are comparedwith the pre-industrial development of atmospheric CO2 known from antarctic ice cores. Our results show that pre-industrial CO2 emissions from ALCC have been relevant for the pre-industrial carbon cycle, although before 1750 AD their trace in atmospheric CO2 is obscured by other processes of similar magnitude. After 1750 AD, the situation is different: the steep increase in atmospheric CO2 until 1850 AD—this is before fossil fuel emissions rose to significant values—is to a substantial part explained by growing emissions from ALCC.

Anthropogenically induced changes in twentieth century mineral dust burden and the associated impact on radiative forcing

We investigate the relative importance of climate change (CC) and anthropogenic land cover change (ALCC) for the dust emissions and burden changes between the late nineteenth century and today. For this purpose, the climate-aerosol model ECHAM6-HAM2 is complemented by a new scheme to derive potential dust sources at runtime using the vegetation cover provided by the land component JSBACH of ECHAM6. Dust emissions are computed online using information from the ECHAM6 atmospheric component. This allows us to account for changes in land cover and climate interactively and to distinguish between emissions from natural and agricultural dust sources. For todays climate we find that nearly 10% of dust particles are emitted from agricultural areas. According to our simulations, global annual dust emissions have increased by 25% between the late nineteenth century and today (e.g., from 729 Tg/a to 912 Tg/a). Globally, CC and ALCC (e.g., agricultural expansion) have both contributed to this change (56% and 40%, respectively). There are however large regional differences. For example, change in dust emissions in Africa are clearly dominated by CC. Global dust burden have increased by 24.5% since the late nineteenth century, which results in a clear-sky radiative forcing at top of the atmosphere of −0.14 W/m2. Based on these findings, we recommend that both climate changes and anthropogenic land cover changes should be considered when investigating long-term changes in dust emissions.

Coupled climate–carbon simulations indicate minor global effects of wars and epidemics on atmospheric CO2 between ad 800 and 1850

Historic events such as wars and epidemics have been suggested as explanation for decreases in atmospheric CO2 reconstructed from ice cores because of their potential to take up carbon in forests regrowing on abandoned agricultural land. Here, we use a coupled climate–carbon cycle model to assess the carbon and climate effects of the Mongol invasion (~1200 to ~1380), the Black Death (~1347 to ~1400), the conquest of the Americas (~1519 to ~1700), and the fall of the Ming Dynasty (~1600 to ~1650). We calculate their impact on atmospheric CO2 including the response of the global land and ocean carbon pools. It has been hypothesized that these events have contributed to significant increases in land carbon stocks. However, we find that slow regrowth and delayed emissions from past land cover change allow for small increases of the land biosphere carbon storage only during long-lasting events. The effect of these small increases in land biosphere storage on global CO2 is reduced by the response of the global carbon pools and largely offset by concurrent emissions from the rest of the world. None of these events would therefore have affected the atmospheric CO2 concentration by more than 1 ppm. Only the Mongol invasion could have lowered global CO2, but by an amount too small to be resolved by ice cores.

Robust Identification of Local Biogeophysical Effects of Land-Cover Change in a Global Climate Model

AbstractLand-cover change (LCC) happens locally. However, in almost all simulation studies assessing biogeophysical climate effects of LCC, local effects (due to alterations in a model grid box) are mingled with nonlocal effects (due to changes in wide-ranging climate circulation). This study presents a method to robustly identify local effects by changing land surface properties in selected “LCC boxes” (where local plus nonlocal effects are present), while leaving others unchanged (where only nonlocal effects are present). While this study focuses on the climate effects of LCC, the method presented here is applicable to any land surface process that is acting locally but is capable of influencing wide-ranging climate when applied on a larger scale. Concerning LCC, the method is more widely applicable than methods used in earlier studies. The study illustrates the possibility of validating simulated local effects by comparison to observations on a global scale and contrasts the underlying mechanisms of lo...

The mutual importance of anthropogenically and climate‐induced changes in global vegetation cover for future land carbon emissions in the MPI‐ESM CMIP5 simulations

Based on the Max Planck Institute Earth System Model simulations for the Coupled Model Intercomparison Project Phase 5 and on simulations with the submodel Cbalone we disentangle the influence of natural and anthropogenic vegetation changes on land carbon emissions for the years 1850 until 2300. According to our simulations, climate-induced changes in distribution and productivity of natural vegetation strongly mitigates future carbon (C) emissions from anthropogenic land-use and land cover change (LULCC). Depending on the assumed scenario, the accumulated carbon emissions until the year 2100 are reduced between 22 and 49% and until 2300 between 45 and 261%. The carbon storage due to climate-induced vegetation change is generally stronger under the presence of LULCC. This is because the natural vegetation change can reestablish highly productive extratropical forests that are lost due to LULCC. After stopping anthropogenic vegetation changes in the year 2100 the refilling of depleted C pools on formerly transformed land takes (dependent on the scenario) time scales of centuries.

Separation of the Effects of Land and Climate Model Errors on Simulated Contemporary Land Carbon Cycle Trends in the MPI Earth System Model version 1

AbstractThe capacity of earth system models (ESMs) to make reliable projections of future atmospheric CO2 and climate is strongly dependent on the ability of the land surface model to adequately simulate the land carbon (C) cycle. Defining “adequate” performance of the land model requires an understanding of the contributions of climate model and land model errors to the land C cycle. Here, a benchmarking framework is applied based on significant, observed characteristics of the land C cycle for the contemporary period, for which sufficient evaluation data are available, to test the ability of the JSBACH land surface component of the Max Planck Institute Earth System Model (MPI-ESM) to simulate land C trends. Particular attention is given to the role of potential effects caused by climate biases, and therefore investigation is made of the results of model configurations in which JSBACH is interactively “coupled” to atmosphere and ocean components and of an “uncoupled” configuration, where JSBACH is driven...