Different Pathways to an Early Eocene Climate

The early Eocene was characterised by much higher temperatures and a smaller equator-to-pole surface temperature gradient than today. Comprehensive climate models have been reasonably successful in simulating many features of that climate in the annual average. However, good simulations of the seasonal variations, and in particular the much reduced Arctic land temperature seasonality and associated much warmer winters, have proven more difficult. Further, aside from an increased level of greenhouse gases, it remains unclear what the key processes are that give rise to an Eocene climate, and whether there is a unique combination of factors that leads to agreement with available proxies. Here we use a very flexible General Circulation Model to examine the sensitivity of the modelled climate to differences in CO2 concentration, land surface properties, ocean heat transport, and cloud extent and thickness. Even in the absence of ice or changes in cloudiness, increasing the CO2 concentration leads to a polar-amplified surface temperature change because of increased water vapour and the lack of convection at high latitudes. Additional low clouds over Arctic land generally decreases summer temperatures and, except at very high CO2 levels, increases winter temperatures, thus helping achieve an Eocene climate. An increase in the land surface heat capacity, plausible given large changes in vegetation and landscape, also decreases the Arctic land seasonality. In general, various different combinations of factors -- high CO2 levels, changes in low-level clouds, and an increase in land surface heat capacity -- can lead to a simulation consistent with current proxy data.

Changes in Body Surface Temperature Associated with High-Speed Treadmill Exercise in Beagle Dogs Measured by Infrared Thermography

Evaluation of body surface temperature change in response to exercise is important for monitoring physiological status. The aim of the study was to assess the influence of high-speed treadmill exercise on body surface temperature using infrared thermography (IRT) in selected body regions of healthy Beagle dogs, taking into account gait and recovery time. Thermographic images of the dogs were taken before exercise (BE), after walk (AW), after trot (AT), after canter (AC), just after second walk (JAE), 5 min after exercise (5 AE), 15 min after exercise (15 AE), 30 min after exercise (30 AE), 45 min after exercise (45 AE), and 120 min after exercise (120 AE). Body surface temperature was measured at the neck, shoulder, upper forearm, back, chest, croup, and thigh. Statistical analysis indicated the highest temperature at the upper forearm, shoulder, and thigh, and the lowest on the croup, back, and neck. The peak values of surface temperature in all ROIs were at AC and JAE and the lowest at 120 AE. The study demonstrated that body surface temperature was influenced by high-speed physical exercise on a treadmill and IRT was a viable imaging modality that provided temperature data from specific body regions. The proximal forelimb and hindlimb were the most influenced by exercise.

Cloud Feedbacks from CanESM2 to CanESM5.0 and their influence on climate sensitivity

The newest iteration of the Canadian Earth System Model (CanESM5.0.3) has an effective climate sensitivity (EffCS) of 5.65 K, which is a 54 % increase relative to the model's previous version (CanESM2 – 3.67 K), and the highest sensitivity of all current models participating in the sixth phase of the coupled model inter-comparison project (CMIP6). Here, we explore the underlying causes behind CanESM5's increased EffCS via comparison of forcing and feedbacks between CanESM2 and CanESM5. We find only modest differences in radiative forcing as a response to CO2 between model versions. We find small increases in the surface albedo and longwave cloud feedback, as well as a substantial increase in the SW cloud feedback in CanESM5. Through the use of cloud area fraction output and cloud radiative kernels, we find that more positive low and non-low shortwave cloud feedbacks – particularly with regards to low clouds across the equatorial Pacific, as well as subtropical and extratropical free troposphere cloud optical depth – are the dominant contributors to CanESM5's increased climate sensitivity. Additional simulations with prescribed sea surface temperatures reveal that the spatial pattern of surface temperature change exerts controls on the magnitude and spatial distribution of low-cloud fraction response but does not fully explain the increased EffCS in CanESM5. The results from CanESM5 are consistent with increased EffCS in several other CMIP6 models, which has been primarily attributed to changes in shortwave cloud feedbacks.

Summer and winter precipitation in East Asia scale with global warming at different rates

Future changes of regional precipitation are of great scientific and societal interests. Large uncertainties still exist in their projections by models. Mechanistic understanding is therefore necessary. Here we demonstrate robust features of the percentage change of precipitation normalized to surface temperature change (%/K) under global warming, referred to as scaling of precipitation with temperature in East Asia. We find that land precipitation in the summer scales at ~3%/K, well below the scaling rate of the Clausius-Clapeyron relationship for atmospheric water vapor content, but the scaling in winter is comparable to the Clausius-Clapeyron scaling at ~7%/K. By using moisture budget analysis of model simulations, we show that this scaling and the seasonal contrast can be clearly attributed to the robust climate changes of steeping moisture gradient, weakening westerly jets, and increasing hydrological amplitude of atmospheric eddies.

Observational constraints reduce estimates of the global mean climate relevance of black carbon

How emissions of black carbon (BC) aerosols affect the climate is still uncertain, due to incomplete knowledge of its sources, optical properties and atmospheric processes such as transport, removal and impact on clouds. Here we constrain simulations from four climate models with observations of atmospheric BC concentrations and absorption efficiency, and the most recent emission inventories, to show that the current global mean surface temperature change from anthropogenic BC emissions is likely to be weak at +0.03 ±0.02K. Atmospheric rapid adjustment processes are found to reduce the top of atmosphere radiative imbalance relative to instantaneous radiative forcing (direct aerosol effect) by almost 50% as a multi-model mean. Furthermore, constraining the models to reproduce observational estimates of the atmospheric vertical profile reduces BC effective radiative forcing to 0.08 W m-2, a value more than 50% lower than in unconstrained simulations. Our results imply a need to revisit commonly used climate metrics such as the global warming potential of BC. This value (for a 100-year time horizon) reduces from 680 when neglecting rapid adjustments and using an unconstrained BC profile to our best estimate of 160 ±120.

The climate impact of COVID-19-induced contrail changes

The COVID-19 pandemic caused significant economic disruption in 2020 and severely impacted air traffic. We use a state-of-the-art Earth system model and ensembles of tightly constrained simulations to evaluate the effect of the reductions in aviation traffic on contrail radiative forcing and climate in 2020. In the absence of any COVID-19-pandemic-caused reductions, the model simulates a contrail effective radiative forcing (ERF) of 62 ± 59 mW m−2 (2 standard deviations). The contrail ERF has complex spatial and seasonal patterns that combine the offsetting effect of shortwave (solar) cooling and longwave (infrared) heating from contrails and contrail cirrus. Cooling is larger in June–August due to the preponderance of aviation in the Northern Hemisphere, while warming occurs throughout the year. The spatial and seasonal forcing variations also map onto surface temperature variations. The net land surface temperature change due to contrails in a normal year is estimated at 0.13 ± 0.04 K (2 standard deviations), with some regions warming as much as 0.7 K. The effect of COVID-19 reductions in flight traffic decreased contrails. The unique timing of such reductions, which were maximum in Northern Hemisphere spring and summer when the largest contrail cooling occurs, means that cooling due to fewer contrails in boreal spring and fall was offset by warming due to fewer contrails in boreal summer to give no significant annual averaged ERF from contrail changes in 2020. Despite no net significant global ERF, because of the spatial and seasonal timing of contrail ERF, some land regions would have cooled slightly (minimum −0.2 K) but significantly from contrail changes in 2020. The implications for future climate impacts of contrails are discussed.