The Warming papers: the scientific foundation for the climate change forecast

The Earth energy imbalance (EEI) is a fundamental climate variable that characterizes the energy state of the climate system. When integrated over multiple years, EEI estimates provide the net energy gain (or loss) by the climate system. In addition, measuring accurately the EEI along with surface temperature and atmospheric composition is essential to separate the role of different radiative forcing from the role of feedbacks on the global energy budget enabling further to constraint effective and equilibrium climate sensitivities. In this presentation I review the current EEI observing system performance and uncertainty. I intercompare the different EEI datasets, originating from in-situ and space-based observing systems to evaluate their differences and to assess their uncertainty. Since 2000 the Clouds and the Earth’s Radiant Energy System (CERES) project provides satellite-based observations of the Earth radiation budget and the EEI with the highest precision (±0.3W.m-2 -1s- on a monthly basis). Nevertheless, because of limitation in the absolute calibration of CERES radiometers the CERES final product needs a bias correction (of about ±2.5W.m-2 -1s-) on the EEI mean. The current best approach to estimating the mean EEI is to estimate the ocean heat uptake (OHU)  which represent 89% of the energy storage  due to the EEI.  Today, the OHU can be derived with the highest accuracy (±0.18W.m-2 -1s- on the mean OHU), from in situ ocean temperature measured by Argo or from the thermal expansion estimated by the difference between satellite altimetry sea level and ocean mass from GRACE. On 2-yr and longer time scales, OHU and CERES EEI estimates show good agreement in EEI variability. But OHU approaches cannot resolve the EEI variability below 1 yr because the energy gain (or loss) induced by EEI over such small time-scales is of the same order of magnitude as the global exchanges of energy between the atmosphere and the ocean. The different EEI measurements have enabled since 2005 a robust estimate of the mean EEI of +0.75±0.18W.m-2 that is essentially due to anthropogenic emissions of greenhouse gases (GHG). They have also allowed to detect a significantly positive trend in EEI of 0.4±0.3W.m-2 per decade, leading to a doubling of the EEI during the past 20 years in response to continued increases in GHG emissions combined with decreases in aerosol emissions. In addition, on interannual time scales, they showed that the variability in EEI is mostly sensitive to low cloud variability, with ENSO controlling the ±0.5W.m-2 variability on the 4-7yr time scale.  Today, new scientific challenges related to EEI are emerging like the closure of the energy budget from top of the atmosphere to the bottom of the ocean at monthly to decadal time scales, the estimate of the current effective climate sensitivity, the monitoring of the physical climate system response to GHG mitigation policies and others. These new challenges lead to new requirements on the EEI observing system ranging from sustained continuity to higher precision and accuracy. I discuss briefly the need to refine these requirements and some opportunities to meet them in the future.

One important question about the CO2‐climate connection is how increasing atmospheric pCO2 levels affect climate and thereby the mass balance of continental ice sheets. A record of atmospheric CO2 variations over the last 160,000 years has recently been constructed by analyzing the trapped gas in the Vostok ice core [Barnola et al., 1987]. The relationship between changes in atmospheric CO2 and the size of the continental ice sheets has been difficult to ascertain because the CO2 record is obtained from ice cores while the ice volume record has been constructed from the stable isotopic composition of biogenic CaCO3 in deep‐sea sediment cores. In order to compare these two records in a more precise manner, we present a record of the isotopic composition (δ18O) of atmospheric O2 trapped in the Vostok ice core, and propose that it may be considered a proxy for the δ18O of seawater and hence ice volume. Having a record of atmospheric CO2 along with a continental ice volume proxy in trapped air in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events. Our results suggest that, during the penultimate glacial termination, atmospheric CO2 began to increase at least 3 kyr before the initial introduction of meltwater to the oceans. Possible errors in the relative age‐depth curve of the Vostok ice core and uncertainties in the influence of biological and hydrologic processes on the δ18O of atmospheric O2 introduce some uncertainty into our conclusions. However, our results are in general agreement with the observed phase relationship between atmospheric CO2 and ice volume [Imbrie et al., 1984; Imbrie et al., 1989] (inferred from changes in the δ13C difference between contemporaneous planktonic and benthic forams deposited in deep‐sea cores and the SPECMAP ice volume record).

Plants are expected to respond to rising levels of atmospheric carbon dioxide by using water more efficiently. Direct evidence of this has been obtained from forests, but the size of the effect will prompt debate. See Letter p.324 Theory suggests that rising atmospheric CO2 concentrations should increase the efficiency with which plants use water, but the actual magnitude of this effect in natural forest ecosystems remains unknown. An analysis of long-term measurements of carbon and water fluxes from forest research sites across the Northern Hemisphere has identified an unexpectedly large increase in water-use efficiency during the past two decades, coinciding with an increase of atmospheric CO2 from 350 to 400 parts per million. This trend is often accompanied by concurrent increases in rates of photosynthetic uptake and carbon sequestration. The authors suggest partial closure of stomata — to maintain constant CO2 concentrations in the plant leaves — as the most likely explanation for the observed trend in water-use efficiency. The results are inconsistent with current theory and terrestrial biosphere models.

The long-term storage of carbon dioxide in soils, vegetation, oceans, and geological formations, with the objective of reducing or postponing global warming, is known as carbon sequestration. It reduces the amount of greenhouse gas emitted by various human activities. Scientists estimate that there is a direct correlation between rising global temperatures and atmospheric carbon dioxide levels, with the atmosphere containing 30% more carbon today than it did 150 years ago. One recommended approach to reducing atmospheric carbon dioxide is to increase carbon storage globally through a variety of means. To slow the net rate of increase in atmospheric CO2, carbon sequestration involves storing CO2 in long-lived global pools found in forests, oceans, biomass, and geological strata. This process is crucial for the maintenance of the global carbon cycle. To combat climate change, the global community must lower carbon emissions and increase net carbon sequestration. These methods can reduce the risks associated with climate change, but it is important to weigh the advantages and disadvantages of each suggested carbon sequestration strategy. The present study attempts to discuss various methods of carbon sequestration to reduce global warming.

Mathematical modeling of vehicle carbon dioxide emissions

<p><strong>Authors:</strong></p><p>Smrati Gupta<sup>1,2</sup>*, Yogesh K. Tiwari<sup>1</sup>, J. V. Revadekar<sup>1</sup>, Pramit Kumar Deb Burman<sup>1</sup>, Supriyo Chakraborty<sup>1</sup>, Palingamoorthy Gnanamoorthy<sup>3,4</sup></p><p><sup>1</sup>Indian Institute of Tropical Meteorology, Pune, Ministry of Earth Sciences, Govt. of India</p><p><sup>2</sup>Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi, India</p><p><sup>3</sup>Coastal Systems Research, M. S. Swaminathan Research Foundation, Chennai, India</p><p><sup>4</sup>Key Laboratory of Tropical Forest Ecology, Chinese Academy of Sciences, Menglun, China</p><p><strong>Abstract            </strong></p><p>A significant amount of the major greenhouse gas, carbon dioxide (CO<sub>2</sub>), released into the atmosphere is sequestered by the terrestrial biosphere. Climatic parameters such as temperature, precipitation, soil moisture, etc., modulate this sequestration capacity or sink in varied limits. A little information is available on the impact of extreme temperatures on the terrestrial biosphere sequestration of atmospheric CO<sub>2</sub>. This study explores the modulation in the terrestrial sink of CO<sub>2</sub> caused by the frequently occurring extreme temperature phenomenon such as heatwaves over the Indian domain. Heatwaves are extreme temperature phenomena extending from the North-west Indian region towards the south-east region, occurring primarily in the pre-monsoon season of March-May (MAM), sometimes prolonged until June. The high intensity and duration of heatwaves during the season lead to the loss of human work capacity, health, economic losses, and even lives. The year 2015 witnessed one of the dreadful heatwave events in recent years, resulting in the loss of more than 2500 human lives within a season owing to heatwaves. The frequency and intensity of heatwaves are projected to increase further soon globally, including India, in the light of global climate change. It is not only of concern for human resources.</p><p>From the biosphere perspective, the terrestrial sink of CO<sub>2</sub> has also been studied to get affected by heatwaves. Temperature is one of the prime factors responsible for photosynthesis and ultimately for the available atmospheric CO<sub>2</sub> fixation by the plants. As such, the CO<sub>2</sub> fixation by the biosphere is affected during MAM season due to limited reduced soil moisture in this hot and dry season, leading to higher atmospheric CO<sub>2</sub> concentrations. In this study, we examine the sub-seasonal variability in the atmospheric CO<sub>2</sub> observed within MAM, driven by subdued fixation by ecosystems in the presence of extreme temperature phenomena like heatwaves. Here, available observations of CO<sub>2</sub> flux or Net Ecosystem Exchange (NEE) flux from MetFlux India Project funded by the Ministry of Earth Sciences, India, studied in conjunction with the retrieved atmospheric and columnar CO<sub>2</sub> concentrations from instruments aboard Atmospheric Infrared Sounder and Orbiting Carbon Observatory-2 satellites during the heatwave period of the year 2015. Our results suggest during a heatwave period, there is an initial increase in carbon uptake by the ecosystem with the temperature rise. But a further rise in temperature after some critical high temperature (~ 32 ͦ C) tends to reduce CO<sub>2</sub> uptake compared to the non-heatwave period of the same season. The satellite retrievals also noticed an increase in atmospheric CO<sub>2</sub> concentrations by 2-3 ppm during the heatwave period. The impact and feedback of heatwaves on the biospheric component of the carbon cycle is one of the significant outcomes of this study.</p>

The recent publication of the summary for policy makers by Working Group I of the Intergovernmental Panel on Climate Change (IPCC) [1] has injected a renewed sense of urgency to address climate change. It is therefore timely to review the notion of preventing 'dangerous anthropogenic interference with the climate system' as put forward in the United Nations Framework Convention on Climate Change (UNFCCC). The article by Danny Harvey in this issue [2] offers a fresh perspective by rephrasing the concept of 'dangerous interference' as a problem of risk assessment. As Harvey points out, identification of 'dangerous interference' does not require us to know with certainty that future climate change will be dangerous—an impossible task given that our knowledge about future climate change includes uncertainty. Rather, it requires the assertion that interference would lead to a significant probability of dangerous climate change beyond some risk tolerance, and therefore would pose an unacceptable risk.

Antarctic ice cores are a preferred climate archive to study global carbon cycle changes at multi-centennial timescales as they provide the only direct reconstructions of past atmospheric CO2 changes. Here we present a new atmospheric CO2 record from the EPICA Dome C ice core spanning Termination III (TIII) and Marine Isotope Stage 7 (MIS 7) (~260-190 ka). 203 ice samples were measured using a ball mill dry extraction system and gas chromatography at IGE. With a temporal resolution of about 300 years on average, our new record improves by a factor of three the existing CO2 record that had been measured on the Vostok ice core over this time interval. Based on our new record, we identified seven centennial-scale releases of atmospheric CO2, also referred as Carbon Dioxide Jumps (CDJ). Combining these new results with previously published ones, we evidenced that 18 of the 22 CDJs identified over the past 500 thousand years occurred under a context of high obliquity. New simulations performed with the LOVECLIM model, an Earth system model of intermediate complexity, point toward both the continental biosphere and the Southern Ocean as the two main carbon sources during CDJs connected to Heinrich events. Notably, the continental biosphere appears to be the obliquity-dependent CO2 source for these rapid events. For the first time, we demonstrate that the long-term external forcing directly impacts past abrupt atmospheric CO2 variations.

Plants respond to changes in atmospheric carbon dioxide levels by regulating the number of stomata in their leaves. In his reconstruction of a continuous, 300-million-year record of atmospheric CO2, Retallack bases his curve on stomatal counts of fossil plant cuticles taken from published micrographs. However, the preservation of cuticles from Permian times is generally too fragmentary for the stomatal index to be reliably determined, the micrographs used could have biased the results, and there are important errors in the supplementary data - all of which cast doubt on the Permian part of Retallack's record.

Direct evidence of past atmospheric CO2 changes has been extended to the past 160,000 years from the Vostok ice core. These changes are most notably an inherent phenomenon of change between glacial and interglacial periods. Besides this major 100,000-year cycle, the CO2 record seems to exhibit a cyclic change with a period of some 21,000 years.

Surface flux perturbations (heat, freshwater, and wind) due to an increase of atmospheric CO2 cause significant intermodel spread in ocean heat uptake; however, the mechanism underlying their impact is not very well understood. Here, we use ocean model experiments to isolate the impact of each perturbation on the ocean heat uptake components, focusing on surface heat flux anomalies caused directly by atmospheric CO2 increase (passive) and indirectly by ocean circulation change (active). Surface heat flux perturbations cause the passive heat uptake, while all the surface flux perturbations influence ocean heat uptake through the active component. While model results have implied that the active component increases ocean heat uptake because of the weakening of the Atlantic meridional overturning circulation (AMOC), we find that it depends more on the shallow circulation change patterns. Surface heat flux perturbation causes most of the AMOC weakening, yet it causes a net global active heat loss (12% of the total uptake), which occurs because the active heat loss in the tropical Pacific through the subtropical cell weakening is greater than the active heat gain in the subpolar Atlantic through AMOC weakening. Freshwater perturbation weakens the AMOC a little more, but increases the subpolar Atlantic heat uptake a great deal through a large weakening of the subpolar gyre, thereby causing a large global active heat gain (34% of the total uptake). Wind perturbation also causes an active heat loss largely through the poleward shift of the Southern Hemisphere subtropical cells.

’s (2012) conclusion that observed climate change is comparableto projections, and in some cases exceeds projections, allows further inferences ifwe can quantify changing climate forcings and compare those with projections.The largest climate forcing is caused by well-mixed long-lived greenhouse gases.Here we illustrate trends of these gases and their climate forcings, and we discussimplications. We focus on quantities that are accurately measured, and we includecomparison with fixed scenarios, which helps reduce common misimpressionsabout how climate forcings are changing.Annual fossil fuel CO

Time‐dependent global warming due to increasing levels of atmospheric carbon dioxide has been estimated by employing an ocean‐land global climate model. Ocean heat capacity is incorporated by means of a global ocean model having a 70 m deep mixed layer, with heat being transported from the mixed layer to deeper waters by eddy diffusion. The time‐dependent increase in atmospheric CO2, from 1860 to 2025, is taken from carbon‐cycle models. The model results suggest that ocean heat capacity will produce a lag in CO2‐induced global warming of about 2 decades. For example, without inclusion of ocean heat capacity the model predicts that an increase in global surface temperature of 1°C, relative to 1860, will occur by 1988. But when ocean heat capacity is included, the 1°C warming is delayed until 2006–2012, this range of times corresponding to no land‐ocean advective coupling (2006) and complete land‐ocean coupling (2012). By 2025, when the assumed atmospheric CO2 content is twice the 1860 value, the model predicts global warming of 1.5°–1.8°C, in contrast to 3.1°C when ocean heat capacity is neglected.

Concurrent changes in climate, atmospheric nitrogen (N) deposition, and increasing levels of atmospheric carbon dioxide (CO2) affect ecosystems in complex ways. The DayCent-Chem model was used to investigate the combined effects of these human-caused drivers of change over the period 1980–2075 at seven forested montane and two alpine watersheds in the United States. Net ecosystem production (NEP) increased linearly with increasing N deposition for six out of seven forested watersheds; warming directly increased NEP at only two of these sites. Warming reduced soil organic carbon storage at all sites by increasing heterotrophic respiration. At most sites, warming together with high N deposition increased nitrous oxide (N2O) emissions enough to negate the greenhouse benefit of soil carbon sequestration alone, though there was a net greenhouse gas sink across nearly all sites mainly due to the effect of CO2 fertilization and associated sequestration by plants. Over the simulation period, an increase in atmospheric CO2 from 350 to 600 ppm was the main driver of change in net ecosystem greenhouse gas sequestration at all forested sites and one of two alpine sites, but an additional increase in CO2 from 600 to 760 ppm produced smaller effects. Warming either increased or decreased net greenhouse gas sequestration, depending on the site. The N contribution to net ecosystem greenhouse gas sequestration averaged across forest sites was only 5–7 % and was negligible for the alpine. Stream nitrate (NO3 −) fluxes increased sharply with N-loading, primarily at three watersheds where initial N deposition values were high relative to terrestrial N uptake capacity. The simulated results displayed fewer synergistic responses to warming, N-loading, and CO2 fertilization than expected. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change.

Limits to Paris compatibility of CO2 capture and utilization