Search

Methane (CH4) has significant absorption bands at wavelengths of 1.7, 2.3, 3.3 and 7.6 μm which absorb incoming solar ‘shortwave’ (SW) radiation and contribute to radiative forcing (RF). A comprehensive quantification of CH4 SW RF is presented using a narrow-band radiative transfer model to calculate spatially and monthly resolved estimates of CH4 SW RF and its impact on longwave (LW) stratospheric temperature adjusted RF (SARF). These new calculations include satellite measurements of CH4 distribution and spectrally varying surface albedo, and include absorption of solar mid-infrared radiation by methane's 7.6 μm band. These factors substantially influence methane's SW effect. For a 750–1,800 ppb perturbation, the all-sky top-of-atmosphere SW instantaneous RF is 0.082 W m−2; at the tropopause it is 0.002 W m−2, considerably smaller than previous estimates. Including the impact of SW absorption on stratospheric temperature increases tropopause SARF by 0.039 W m−2 (or 7%) compared to the LW-only SARF.

Understanding the nature of air parcels that exhibit ice-supersaturation is important because they are the regions of potential formation of both cirrus and aircraft contrails, which affect the radiation balance. Ice-supersaturated air parcels in the upper troposphere and lower stratosphere over the North Atlantic are investigated using Lagrangian trajectories. The trajectory calculations use ERA-Interim data for three winter and three summer seasons, resulting in approximately 200,000 trajectories with ice-supersaturation for each season. For both summer and winter, the median duration of ice-supersaturation along a trajectory is less than 6 hours. 5% of air which becomes ice-supersaturated in the troposphere, and 23% of air which becomes ice-supersaturated in the stratosphere will remain ice-supersaturated for at least 24 hours. Weighting the ice-supersaturation duration with the observed frequency indicates the likely overall importance of the longer duration ice-supersaturated trajectories. Ice-supersaturated air parcels typically experience a decrease in moisture content while ice-supersaturated, suggesting that cirrus clouds eventually form in the majority of such air. A comparison is made between short-lived (less than 24 h) and long-lived (greater than 24 h) ice-supersaturated air flows. For both air flows, ice-supersaturation occurs around the northernmost part of the trajectory. Short-lived ice-supersaturated air flows show no significant differences in speed or direction of movement to subsaturated air parcels. However, long-lived ice-supersaturated air occurs in slower moving air flows, which implies that they are not associated with the fastest moving air through a jet stream.

New calculations of the radiative forcing (RF) are presented for the three main well‐mixed\ud greenhouse gases, methane, nitrous oxide, and carbon dioxide. Methane’s RF is particularly impacted\ud because of the inclusion of the shortwave forcing; the 1750–2011 RF is about 25% higher (increasing from\ud 0.48 W m−2 to 0.61 W m−2) compared to the value in the Intergovernmental Panel on Climate Change (IPCC)\ud 2013 assessment; the 100 year global warming potential is 14% higher than the IPCC value. We present new\ud simplified expressions to calculate RF. Unlike previous expressions used by IPCC, the new ones include the\ud overlap between CO2 and N2O; for N2O forcing, the CO2 overlap can be as important as the CH4 overlap. The\ud 1750–2011 CO2 RF is within 1% of IPCC’s value but is about 10% higher when CO2 amounts reach 2000 ppm, a\ud value projected to be possible under the extended RCP8.5 scenario.

The recent slowdown in the rate of increase in global-mean surface temperature (GMST) has generated extensive discussion, but little attention has been given to the contribution of time-varying trends in greenhouse gas concentrations. We use a simple model approach to quantify this contribution. Between 1985 and 2003, greenhouse gases (including well-mixed greenhouse gases, tropospheric and stratospheric ozone, and stratospheric water vapour from methane oxidation) caused a reduction in GMST trend of around 0.03–0.05 K decade−1 which is around 18%–25% of the observed trend over that period. The main contributors to this reduction are the rapid change in the growth rates of ozone-depleting gases (with this contribution slightly opposed by stratospheric ozone depletion itself) and the weakening in growth rates of methane and tropospheric ozone radiative forcing. Although CO2 is the dominant greenhouse gas contributor to GMST trends, the continued increase in CO2 concentrations offsets only about 30% of the simulated trend reduction due to these other contributors. These results emphasize that trends in non-CO2 greenhouse gas concentrations can make significant positive and negative contributions to changes in the rate of warming, and that they need to be considered more closely in analyses of the causes of such variations.

The precipitation response to radiative forcing (RF) can be decomposed into a fast precipitation response (FPR), which depends on the atmospheric component of RF, and a slow response, which depends on surface temperature change. We present the first detailed climate model study of the FPR due to tropospheric and stratospheric ozone changes. The FPR depends strongly on the altitude of ozone change. Increases below about 3 km cause a positive FPR; increases above cause a negative FPR. The FPR due to stratospheric ozone change is, per unit RF, about 3 times larger than that due to tropospheric ozone. As historical ozone trends in the troposphere and stratosphere are opposite in sign, so too are the FPRs. Simple climate model calculations of the time-dependent total (fast and slow) precipitation change, indicate that ozone's contribution to precipitation change in 2011, compared to 1765, could exceed 50% of that due to CO2 change.

Aircraft flying through cold ice-supersaturated air produce persistent contrails which contribute to the climate impact of aviation. Here, we demonstrate the importance of the weather situation, together with the route and altitude of the aircraft through this, on estimating contrail coverage. The results have implications for determining the climate impact of contrails as well as potential mitigation strategies. Twenty-one years of re-analysis data are used to produce a climatological assessment of conditions favorable for persistent contrail formation between 200 and 300 hPa over the north Atlantic in winter. The seasonal-mean frequency of cold ice-supersaturated regions is highest near 300 hPa, and decreases with altitude. The frequency of occurrence of ice-supersaturated regions varies with large-scale weather pattern; the most common locations are over Greenland, on the southern side of the jet stream and around the northern edge of high pressure ridges. Assuming aircraft take a great circle route, as opposed to a more realistic time-optimal route, is likely to lead to an error in the estimated contrail coverage, which can exceed 50% for westbound north Atlantic flights. The probability of contrail formation can increase or decrease with height, depending on the weather pattern, indicating that the generic suggestion that flying higher leads to fewer contrails is not robust.