SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Intergovernmental Panel on Climate Change

586

We assess the relationship between temperature and global sea-level (GSL) variability over the Common Era through a statistical metaanalysis of proxy relative sea-level reconstructions and tide-gauge data. GSL rose at 0.1 ± 0.1 mm/y (2σ) over 0-700 CE. A GSL fall of 0.2 ± 0.2 mm/y over 1000-1400 CE is associated with ∼0.2 °C global mean cooling. A significant GSL acceleration began in the 19th century and yielded a 20th century rise that is extremely likely (probability [Formula: see text]) faster than during any of the previous 27 centuries. A semiempirical model calibrated against the GSL reconstruction indicates that, in the absence of anthropogenic climate change, it is extremely likely ([Formula: see text]) that 20th century GSL would have risen by less than 51% of the observed [Formula: see text] cm. The new semiempirical model largely reconciles previous differences between semiempirical 21st century GSL projections and the process model-based projections summarized in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report.

Concepts: Statistics, Climate change, 20th century, 21st century, Centuries, Intergovernmental Panel on Climate Change, Global warming, 19th century

122

Sea level has been steadily rising over the past century, predominantly due to anthropogenic climate change. The rate of sea level rise will keep increasing with continued global warming, and, even if temperatures are stabilized through the phasing out of greenhouse gas emissions, sea level is still expected to rise for centuries. This will affect coastal areas worldwide, and robust projections are needed to assess mitigation options and guide adaptation measures. Here we combine the equilibrium response of the main sea level rise contributions with their last century’s observed contribution to constrain projections of future sea level rise. Our model is calibrated to a set of observations for each contribution, and the observational and climate uncertainties are combined to produce uncertainty ranges for 21st century sea level rise. We project anthropogenic sea level rise of 28-56 cm, 37-77 cm, and 57-131 cm in 2100 for the greenhouse gas concentration scenarios RCP26, RCP45, and RCP85, respectively. Our uncertainty ranges for total sea level rise overlap with the process-based estimates of the Intergovernmental Panel on Climate Change. The “constrained extrapolation” approach generalizes earlier global semiempirical models and may therefore lead to a better understanding of the discrepancies with process-based projections.

Concepts: Carbon dioxide, Climate change, Ocean, 21st century, Intergovernmental Panel on Climate Change, Greenhouse gas, Global warming, Attribution of recent climate change

114

The latest Intergovernmental Panel on Climate Change Assessment Report widened the equilibrium climate sensitivity (ECS) range from 2° to 4.5°C to an updated range of 1.5° to 4.5°C in order to account for the lack of consensus between estimates based on models and historical observations. The historical ECS estimates range from 1.5° to 3°C and are derived assuming a linear radiative response to warming. A Bayesian methodology applied to 24 models, however, documents curvature in the radiative response to warming from an evolving contribution of interannual to centennial modes of radiative response. Centennial modes display stronger amplifying feedbacks and ultimately contribute 28 to 68% (90% credible interval) of equilibrium warming, yet they comprise only 1 to 7% of current warming. Accounting for these unresolved centennial contributions brings historical records into agreement with model-derived ECS estimates.

Concepts: Scientific method, Mode, Ecology, Climate change, History, Intergovernmental Panel on Climate Change, Contributions, Gregorian chant

64

Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century, challenging the prevailing view that anthropogenic forcing causes climate warming. Various mechanisms have been proposed for this hiatus in global warming, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970-2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase.

Concepts: Earth, Climate, Climate change, Intergovernmental Panel on Climate Change, Solar variation, Global warming, Anthropogenic, Attribution of recent climate change

35

Antarctic terrestrial biodiversity occurs almost exclusively in ice-free areas that cover less than 1% of the continent. Climate change will alter the extent and configuration of ice-free areas, yet the distribution and severity of these effects remain unclear. Here we quantify the impact of twenty-first century climate change on ice-free areas under two Intergovernmental Panel on Climate Change (IPCC) climate forcing scenarios using temperature-index melt modelling. Under the strongest forcing scenario, ice-free areas could expand by over 17,000 km(2) by the end of the century, close to a 25% increase. Most of this expansion will occur in the Antarctic Peninsula, where a threefold increase in ice-free area could drastically change the availability and connectivity of biodiversity habitat. Isolated ice-free areas will coalesce, and while the effects on biodiversity are uncertain, we hypothesize that they could eventually lead to increasing regional-scale biotic homogenization, the extinction of less-competitive species and the spread of invasive species.

Concepts: Conservation biology, Ecology, Ecosystem, Extinction, Intergovernmental Panel on Climate Change, Antarctica, Invasive species, Global warming

31

Atmospheric black carbon has long been recognized as a public health and environmental concern. More recently, black carbon has been identified as a major, ongoing contributor to anthropogenic climate change, thus making historical emission inventories of black carbon an essential tool for assessing past climate sensitivity and modeling future climate scenarios. Current estimates of black carbon emissions for the early industrial era have high uncertainty, however, because direct environmental sampling is sparse before the mid-1950s. Using photometric reflectance data of >1,300 bird specimens drawn from natural history collections, we track relative ambient concentrations of atmospheric black carbon between 1880 and 2015 within the US Manufacturing Belt, a region historically reliant on coal and dense with industry. Our data show that black carbon levels within the region peaked during the first decade of the 20th century. Following this peak, black carbon levels were positively correlated with coal consumption through midcentury, after which they decoupled, with black carbon concentrations declining as consumption continued to rise. The precipitous drop in atmospheric black carbon at midcentury reflects policies promoting burning efficiency and fuel transitions rather than regulating emissions alone. Our findings suggest that current emission inventories based on predictive modeling underestimate levels of atmospheric black carbon for the early industrial era, suggesting that the contribution of black carbon to past climate forcing may also be underestimated. These findings build toward a spatially dynamic emission inventory of black carbon based on direct environmental sampling.

Concepts: Carbon dioxide, Climate, Climate change, Methane, Fossil fuel, Coal, Intergovernmental Panel on Climate Change, Global warming

25

Significant climate risks are associated with a positive carbon-temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the “cost” of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse-response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange.

Concepts: Carbon dioxide, Climate, Ecosystem, Climate change, Methane, Intergovernmental Panel on Climate Change, Biome, Greenhouse gas

15

Results are presented from a survey held among 1868 scientists studying various aspects of climate change, including physical climate, climate impacts and mitigation. The survey was unique in its size, broadness and level of detail. Consistent with other research, we found that, as the level of expertise in climate science grew, so too did the level of agreement on anthropogenic causation. 90% of respondents with more than 10 climate-related peer-reviewed publications (about half of all respondents), explicitly agreed with anthropogenic greenhouse gases (GHGs) being the dominant driver of recent global warming. The respondents' quantitative estimate of the GHG contribution appeared to strongly depend on the judgment or knowledge of the cooling effect of aerosols. The phrasing of the IPCC attribution statement in its fourth assessment report (AR4) - providing a lower limit for the isolated GHG contribution - may have led to an underestimation of the GHG influence on recent warming. The phrasing was improved in AR5. We also report on the respondents' views on other factors contributing to global warming; of these Land Use and Land Cover Change (LULCC) was considered the most important. Respondents who characterized human influence on climate as insignificant, reported having had the most frequent media coverage regarding their views on climate change.

Concepts: Climate change, Natural gas, Intergovernmental Panel on Climate Change, Greenhouse gas, Global warming, Anthropogenic, IPCC Fourth Assessment Report, Special Report on Emissions Scenarios

11

We propose a transparent climate debt index incorporating both methane (CH4) and carbon dioxide (CO2) emissions. We develop national historic emissions databases for both greenhouse gases to 2005, justifying 1950 as the starting point for global perspectives. We include CO2 emissions from fossil sources [CO2(f)], as well as, in a separate analysis, land use change and forestry. We calculate the CO2(f) and CH4 remaining in the atmosphere in 2005 from 205 countries using the Intergovernmental Panel on Climate Change’s Fourth Assessment Report impulse response functions. We use these calculations to estimate the fraction of remaining global emissions due to each country, which is applied to total radiative forcing in 2005 to determine the combined climate debt from both greenhouse gases in units of milliwatts per square meter per country or microwatts per square meter per person, a metric we term international natural debt (IND). Australia becomes the most indebted large country per capita because of high CH4 emissions, overtaking the United States, which is highest for CO2(f). The differences between the INDs of developing and developed countries decline but remain large. We use IND to assess the relative reduction in IND from choosing between CO2(f) and CH4'control measures and to contrast the imposed versus experienced health impacts from climate change. Based on 2005 emissions, the same hypothetical impact on world 2050 IND could be achieved by decreasing CH4 emissions by 46% as stopping CO2 emissions entirely, but with substantial differences among countries, implying differential optimal strategies. Adding CH4 shifts the basic narrative about differential international accountability for climate change.

Concepts: Carbon dioxide, Climate change, Natural gas, Methane, Fossil fuel, Intergovernmental Panel on Climate Change, Greenhouse gas, Global warming

11

Despite its enormous cost, large-scale carbon capture and storage (CCS) is considered a viable strategy for significantly reducing CO(2) emissions associated with coal-based electrical power generation and other industrial sources of CO(2) [Intergovernmental Panel on Climate Change (2005) IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change, eds Metz B, et al. (Cambridge Univ Press, Cambridge, UK); Szulczewski ML, et al. (2012) Proc Natl Acad Sci USA 109:5185-5189]. We argue here that there is a high probability that earthquakes will be triggered by injection of large volumes of CO(2) into the brittle rocks commonly found in continental interiors. Because even small- to moderate-sized earthquakes threaten the seal integrity of CO(2) repositories, in this context, large-scale CCS is a risky, and likely unsuccessful, strategy for significantly reducing greenhouse gas emissions.

Concepts: Carbon dioxide, Climate change, Natural gas, Coal, Intergovernmental Panel on Climate Change, Greenhouse gas, Carbon capture and storage, Geoengineering