Most present-generation climate models simulate an increase in global-mean surface temperature (GMST) since 1998, whereas observations suggest a warming hiatus. It is unclear to what extent this ...mismatch is caused by incorrect model forcing, by incorrect model response to forcing or by random factors. Here we analyse simulations and observations of GMST from 1900 to 2012, and show that the distribution of simulated 15-year trends shows no systematic bias against the observations. Using a multiple regression approach that is physically motivated by surface energy balance, we isolate the impact of radiative forcing, climate feedback and ocean heat uptake on GMST--with the regression residual interpreted as internal variability--and assess all possible 15- and 62-year trends. The differences between simulated and observed trends are dominated by random internal variability over the shorter timescale and by variations in the radiative forcings used to drive models over the longer timescale. For either trend length, spread in simulated climate feedback leaves no traceable imprint on GMST trends or, consequently, on the difference between simulations and observations. The claim that climate models systematically overestimate the response to radiative forcing from increasing greenhouse gas concentrations therefore seems to be unfounded.
Research reported during the past decade has shown that global warming is roughly proportional to the total amount of carbon dioxide released into the atmosphere. This makes it possible to estimate ...the remaining carbon budget: the total amount of anthropogenic carbon dioxide that can still be emitted into the atmosphere while holding the global average temperature increase to the limit set by the Paris Agreement. However, a wide range of estimates for the remaining carbon budget has been reported, reducing the effectiveness of the remaining carbon budget as a means of setting emission reduction targets that are consistent with the Paris Agreement. Here we present a framework that enables us to track estimates of the remaining carbon budget and to understand how these estimates can improve over time as scientific knowledge advances. We propose that application of this framework may help to reconcile differences between estimates of the remaining carbon budget and may provide a basis for reducing uncertainty in the range of future estimates.
The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on ...the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (±0.23) W/sq. m, comprised of 1.81 (±0.09) W/sq. m from CO2, 1.08 (± 0.21) W/sq. m from other well-mixed greenhouse gases, −1.01 (± 0.23) W/sq. m from aerosols and −0.09 (±0.13) W/sq. m from land use change. Quoted uncertainties are 1 standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W/sq. m. The majority of the remaining 0.21 W/sq. m is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol–cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from −0.63 to −1.37 W/sq. m, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4×CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming.
Recent attempts to diagnose equilibrium climate sensitivity (ECS) from changes in Earth's energy budget point toward values at the low end of the Intergovernmental Panel on Climate Change Fifth ...Assessment Report (AR5)'s likely range (1.5-4.5 K). These studies employ observations but still require an element of modeling to infer ECS. Their diagnosed effective ECS over the historical period of around 2 K holds up to scrutiny, but there is tentative evidence that this underestimates the true ECS from a doubling of carbon dioxide. Different choices of energy imbalance data explain most of the difference between published best estimates, and effective radiative forcing dominates the overall uncertainty. For decadal analyses the largest source of uncertainty comes from a poor understanding of the relationship between ECS and decadal feedback. Considerable progress could be made by diagnosing effective radiative forcing in models.
The phrasing of the first of three questions motivating CMIP6 – “How does the Earth system respond to forcing?” – suggests that forcing is always well-known, yet the radiative forcing to which this ...question refers has historically been uncertain in coordinated experiments even as understanding of how best to infer radiative forcing has evolved. The Radiative Forcing Model Intercomparison Project (RFMIP) endorsed by CMIP6 seeks to provide a foundation for answering the question through three related activities: (i) accurate characterization of the effective radiative forcing relative to a near-preindustrial baseline and careful diagnosis of the components of this forcing; (ii) assessment of the absolute accuracy of clear-sky radiative transfer parameterizations against reference models on the global scales relevant for climate modeling; and (iii) identification of robust model responses to tightly specified aerosol radiative forcing from 1850 to present. Complete characterization of effective radiative forcing can be accomplished with 180 years (Tier 1) of atmosphere-only simulation using a sea-surface temperature and sea ice concentration climatology derived from the host model's preindustrial control simulation. Assessment of parameterization error requires trivial amounts of computation but the development of small amounts of infrastructure: new, spectrally detailed diagnostic output requested as two snapshots at present-day and preindustrial conditions, and results from the model's radiation code applied to specified atmospheric conditions. The search for robust responses to aerosol changes relies on the CMIP6 specification of anthropogenic aerosol properties; models using this specification can contribute to RFMIP with no additional simulation, while those using a full aerosol model are requested to perform at least one and up to four 165-year coupled ocean–atmosphere simulations at Tier 1.
Committed warming describes how much future warming can be expected from historical emissions due to inertia in the climate system. It is usually defined in terms of the level of warming above the ...present for an abrupt halt of emissions. Owing to socioeconomic constraints, this situation is unlikely, so we focus on the committed warming from present-day fossil fuel assets. Here we show that if carbon-intensive infrastructure is phased out at the end of its design lifetime from the end of 2018, there is a 64% chance that peak global mean temperature rise remains below 1.5 °C. Delaying mitigation until 2030 considerably reduces the likelihood that 1.5 °C would be attainable even if the rate of fossil fuel retirement was accelerated. Although the challenges laid out by the Paris Agreement are daunting, we indicate 1.5 °C remains possible and is attainable with ambitious and immediate emission reduction across all sectors.
Radiative forcing is a useful tool for predicting equilibrium global temperature change. However, it is not so useful for predicting global precipitation changes, as changes in precipitation strongly ...depend on the climate change mechanism and how it perturbs the atmospheric and surface energy budgets. Here a suite of climate model experiments and radiative transfer calculations are used to quantify and assess this dependency across a range of climate change mechanisms. It is shown that the precipitation response can be split into two parts: a fast atmospheric response that strongly correlates with the atmospheric component of radiative forcing, and a slower response to global surface temperature change that is independent of the climate change mechanism, ∼2‐3% per unit of global surface temperature change. We highlight the precipitation response to black carbon aerosol forcing as falling within this range despite having an equilibrium response that is of opposite sign to the radiative forcing and global temperature change.
Biochar has potential to sequester carbon in soils and simultaneously improve soil quality and plant growth. More understanding of biochar variation is needed to optimise these potential benefits. ...Slow pyrolysis at 600 °C was undertaken to determine how yields and characteristics of biochars differ when produced from eight different agricultural residues. Biochar properties such as carbon content, surface area, pH, ultimate and proximate analysis, nutrient and metal content and the R50 recalcitrance index were determined. Significant variations seen in biochar characteristics were attributed to feedstock variation since pyrolysis conditions were constant. Biochar yields varied from 28% to 39%. Average carbon content was 51%. Ash content of both feedstocks and biochars were correlated with biochar carbon content. Macronutrients were concentrated during pyrolysis, but biochar macronutrient content was low in comparison to biochars produced from more nutrient rich feedstocks. Most biochars were slightly alkaline, ranging from pH 6.1 to pH 11.6. pH was correlated with biochar K content. Aromaticity was increased with pyrolysis, shown by a reduction in biochar H/C and O/C ratios relative to feedstock values. The R50 recalcitrance index showed biochars to be either class 2 or class 3. Biochar carbon sequestration potential was 21.3%–32.5%. The R50 recalcitrance index is influenced by the presence of alkali metals in the biochar which may lead to an under-estimation of biochar stability. The residues assessed here, at current global availability, could produce 373 Mt of biochar. This quantity of biochar has the potential to sequester 0.55 Pg CO2 yr−1 in soils over long time periods.
•Biochars from eight crop residue feedstocks have been characterised in detail.•Variation in biochar recalcitrance and carbon sequestration potential is noted.•High alkali content catalysed the degradation of biochars.•21.3%–32.5% of the feedstock carbon would be stored long-term.•150 Mt C yr−1 (0.55 Pg CO2 eq yr−1) could be sequestered using these crop residues.
Using five climate model simulations of the response to an abrupt quadrupling of CO₂, the authors perform the first simultaneous model intercomparison of cloud feedbacks and rapid radiative ...adjustments with cloud masking effects removed, partitioned among changes in cloud types and gross cloud properties. Upon CO₂ quadrupling, clouds exhibit a rapid reduction in fractional coverage, cloud-top pressure, and optical depth, with each contributing equally to a 1.1 W m−2net cloud radiative adjustment, primarily from shortwave radiation. Rapid reductions in midlevel clouds and optically thick clouds are important in reducing planetary albedo in every model. As the planet warms, clouds become fewer, higher, and thicker, and global mean net cloud feedback is positive in all but one model and results primarily from increased trapping of longwave radiation. As was true for earlier models, high cloud changes are the largest contributor to intermodel spread in longwave and shortwave cloud feedbacks, but low cloud changes are the largest contributor to the mean and spread in net cloud feedback. The importance of the negative optical depth feedback relative to the amount feedback at high latitudes is even more marked than in earlier models. The authors show that the negative longwave cloud adjustment inferred in previous studies is primarily caused by a 1.3 W m−2cloud masking of CO₂ forcing. Properly accounting for cloud masking increases net cloud feedback by 0.3 W m−2K−1, whereas accounting for rapid adjustments reduces by 0.14 W m−2K−1the ensemble mean net cloud feedback through a combination of smaller positive cloud amount and altitude feedbacks and larger negative optical depth feedbacks.
•Phase change materials (PCMs) can passively cool PV panels to increase energy output.•A global numerical analysis of PV energy output with PCM cooling is presented.•The most promising locations for ...PCM cooling are in the tropics.•A relative performance improvement of over 6% is possible in some regions.•A sub-optimal PCM melting temperature still produces a beneficial energy output enhancement.
This paper describes a global analysis to determine the increase in annual energy output attained by a PV system with an integrated phase change material (PCM) layer. The PCM acts as a heat sink and limits the peak temperature of the PV cell thereby increasing efficiency. The simulation uses a one-dimensional energy balance model with ambient temperature, irradiance and wind speed extracted from ERA-Interim reanalysis climate data over a 1.5° longitude×1.5° latitude global grid. The effect of varying the PCM melting temperature from 0°C to 50°C was investigated to identify the optimal melting temperature at each grid location. PCM-enhanced cooling is most beneficial in regions with high insolation and little intra-annual variability in climate. When using the optimal PCM melting temperature, the annual PV energy output increases by over 6% in Mexico and eastern Africa, and over 5% in many locations such as Central and South America, much of Africa, Arabia, Southern Asia and the Indonesian archipelago. In Europe, the energy output enhancement varies between 2% and nearly 5%. In general, high average ambient temperatures correlate with higher optimal PCM melting temperatures. The sensitivity to PCM melting temperature was further investigated at locations where large solar PV arrays currently exist or are planned to be constructed. Significant improvements in performance are possible even when a sub-optimal PCM melting temperature is used. A brief economic assessment based on typical material costs and energy prices shows that PCM cooling is not currently cost-effective for single-junction PV.
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