C. Brühl

Assessment of temperature, trace species, and ozone in chemistry-climate model simulations of the recent past

Simulations of the stratosphere from thirteen coupled chemistry-climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long-term trends are compared for an extended period (1960–2004). Comparisons of polar high-latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long-term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.

Multimodel projections of stratospheric ozone in the 21st century

[1] Simulations from eleven coupled chemistry-climate models (CCMs) employing nearly identical forcings have been used to project the evolution of stratospheric ozone throughout the 21st century. The model-to-model agreement in projected temperature trends is good, and all CCMs predict continued, global mean cooling of the stratosphere over the next 5 decades, increasing from around 0.25 K/decade at 50 hPa to around 1 K/ decade at 1 hPa under the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario. In general, the simulated ozone evolution is mainly determined by decreases in halogen concentrations and continued cooling of the global stratosphere due to increases in greenhouse gases (GHGs). Column ozone is projected to increase as stratospheric halogen concentrations return to 1980s levels. Because of ozone increases in the middle and upper stratosphere due to GHGinduced cooling, total ozone averaged over midlatitudes, outside the polar regions, and globally, is projected to increase to 1980 values between 2035 and 2050 and before lowerstratospheric halogen amounts decrease to 1980 values. In the polar regions the CCMs simulate small temperature trends in the first and second half of the 21st century in midwinter. Differences in stratospheric inorganic chlorine (Cly) among the CCMs are key to diagnosing the intermodel differences in simulated ozone recovery, in particular in the Antarctic. It is found that there are substantial quantitative differences in the simulated Cly, with the October mean Antarctic Cly peak value varying from less than 2 ppb to over 3.5 ppb in the CCMs, and the date at which the Cly returns to 1980 values varying from before 2030 to after 2050. There is a similar variation in the timing of recovery of Antarctic springtime column ozone back to 1980 values. As most models underestimate peak Clynear 2000, ozone recovery in the Antarctic could occur even later, between 2060 and 2070. In the Arctic the column ozone increase in spring does not follow halogen decreases as closely as in the Antarctic, reaching 1980 values before Arctic halogen amounts decrease

Impact of stratospheric ozone on Southern Hemisphere circulation change: A multimodel assessment

The impact of stratospheric ozone on the tropospheric general circulation of the Southern Hemisphere (SH) is examined with a set of chemistry-climate models participating in the Stratospheric Processes and their Role in Climate (SPARC)/Chemistry-Climate Model Validation project phase 2 (CCMVal-2). Model integrations of both the past and future climates reveal the crucial role of stratospheric ozone in driving SH circulation change: stronger ozone depletion in late spring generally leads to greater poleward displacement and intensification of the tropospheric midlatitude jet, and greater expansion of the SH Hadley cell in the summer. These circulation changes are systematic as poleward displacement of the jet is typically accompanied by intensification of the jet and expansion of the Hadley cell. Overall results are compared with coupled models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), and possible mechanisms are discussed. While the tropospheric circulation response appears quasi-linearly related to stratospheric ozone changes, the quantitative response to a given forcing varies considerably from one model to another. This scatter partly results from differences in model climatology. It is shown that poleward intensification of the westerly jet is generally stronger in models whose climatological jet is biased toward lower latitudes. This result is discussed in the context of quasi-geostrophic zonal mean dynamics.

Chemistry-climate model simulations of twenty-first century stratospheric climate and circulation changes

The response of stratospheric climate and circulation to increasing amounts of greenhouse gases (GHGs) and ozone recovery in the twenty-first century is analyzed in simulations of 11 chemistry–climate models using near-identical forcings and experimental setup. In addition to an overall global cooling of the stratosphere in the simulations (0.59 6 0.07 K decade 21 at 10 hPa), ozone recovery causes a warming of the Southern Hemisphere polar lower stratosphere in summer with enhanced cooling above. The rate of warming correlates with the rate of ozone recovery projected by the models and, on average, changes from 0.8 to 0.48 K decade 21 at 100 hPa as the rate of recovery declines from the first to the second half of the century. In the winter northern polar lower stratosphere the increased radiative cooling from the growing abundance of GHGs is, in most models, balanced by adiabatic warming from stronger polar downwelling. In the Antarctic lower stratosphere the models simulate an increase in low temperature extremes required for polar stratospheric cloud (PSC) formation, but the positive trend is decreasing over the twenty-first century in all models. In the Arctic, none of the models simulates a statistically significant increase in Arctic PSCs throughout the twentyfirst century. The subtropical jets accelerate in response to climate change and the ozone recovery produces a westward acceleration of the lower-stratospheric wind over the Antarctic during summer, though this response is sensitive to the rate of recovery projected by the models. There is a strengthening of the Brewer–Dobson

Halogen Occultation Experiment ozone channel validation

The HALogen Occultation Experiment (HALOE) instrument on UARS observes vertical profiles of ozone and other gases of interest for atmospheric chemistry using the solar occultation technique. A broadband radiometer in the 9.6-μm band is used for ozone measurements. Version 17 ozone retrieved by HALOE is intercompared successfully with about 400 profiles of other sounders, including ozonesondes, lidars, balloons, rocketsondes, and other satellites. Usually, the HALOE data are within the error range of the correlative measurements between about 100 and 0.03 mbar atmospheric pressure. Between about 30 and 1 mbar, HALOE agrees typically within 5%, with a tendency to be low. In the first year of data, larger errors sometimes occur in the lower stratosphere due to the necessary correction for Pinatubo aerosol effects, but these differences do not exceed 20%. The data show internal consistency for sunrise and sunset events at the same locations. Some examples of observed ozone distributions, including polar regions, are given.

Climate effects of atmospheric methane

We discuss the role of the concentration increase of atmospheric CH4 (currently ≈ 0.8% yr−1) in climate change, including effects through chemical feedbacks, notably through formation of O3 and stratospheric H2O, and reduction of OH levels, which increases the lifetime of CH4. We present results of simulations with a coupled 1-D radiative-convective model, which includes chemical and radiative processes in the troposphere and stratosphere, and which treats both hemispheres separately. Our results deviate appreciably from those reported in earlier studies; the reason for this is discussed in detail. The current climate forcing by CH4 (excluding indirect chemical effects) is 26 times that of CO2 (calculated on a mole CO2mole CH4 basis). A recently introduced quantity to express the time integrated effect of CH4 relative to that of CO2 over a given time interval is the global warming potential (GW P). We assess that the GW P of CH4, including chemical feedbacks, over a 10 year integration period is 26.9 (mole CO2mole CH4), decreasing to 7.5 for a time horizon of 100 years. Considering that the use of fossil fuels is associated with emissions of both CO2 and CH4, we evaluated the climatic consequences of switching from coal or oil to natural gas. We conclude that, if the fractional gas leakage from production and distribution of natural gas is below 4.3 – 5.7%, a switch from coal to natural gas as energy source would reduce the rate of climate warming. The use of gas would be preferable over the use of oil (from a climate point of view), if the fractional gas leakage is less than 2.4 – 2.9%. The ranges express uncertainties in CH4 releases from coal and oil production.

Multimodel assessment of the upper troposphere and lower stratosphere: Tropics and global trends

The performance of 18 coupled Chemistry Climate Models (CCMs) in the Tropical Tropopause Layer (TTL) is evaluated using qualitative and quantitative diagnostics. Trends in tropopause quantities in the tropics and the extratropical Upper Troposphere and Lower Stratosphere (UTLS) are analyzed. A quantitative grading methodology for evaluating CCMs is extended to include variability and used to develop four different grades for tropical tropopause temperature and pressure, water vapor and ozone. Four of the 18 models and the multi‐model mean meet quantitative and qualitative standards for reproducing key processes in the TTL. Several diagnostics are performed on a subset of the models analyzing the Tropopause Inversion Layer (TIL), Lagrangian cold point and TTL transit time. Historical decreases in tropical tropopause pressure and decreases in water vapor are simulated, lending confidence to future projections. The models simulate continued decreases in tropopause pressure in the 21st century, along with ∼1K increases per century in cold point tropopause temperature and 0.5–1 ppmv per century increases in water vapor above the tropical tropopause. TTL water vapor increases below the cold point. In two models, these trends are associated with 35% increases in TTL cloud fraction. These changes indicate significant perturbations to TTL processes, specifically to deep convective heating and humidity transport. Ozone in the extratropical lowermost stratosphere has significant and hemispheric asymmetric trends. O3 is projected to increase by nearly 30% due to ozone recovery in the Southern Hemisphere (SH) and due to enhancements in the stratospheric circulation. These UTLS ozone trends may have significant effects in the TTL and the troposphere.

Carbon 13 and D kinetic isotope effects in the reactions of CH4 with O(1 D) and OH: New laboratory measurements and their implications for the isotopic composition of stratospheric methane

Measurements of the 13C and D kinetic isotope effects (KIE) in methane, 13CKIE = k(12CH4)/k(13CH4) and DKIE = k(12CH4)/k(12CH3D), in the reactions of these atmospherically important methane isotopomers with O(1D) and OH have been undertaken using mass spectrometry and tunable diode laser absorption spectroscopy to determine isotopic composition. For the carbon kinetic isotope effect in the reaction with the OH radical, 13CKIEOH = 1.0039 (±0.0004, 2σ) was determined at 296 K, which is significantly smaller than the presently accepted value of 1.0054 (±0.0009, 2 σ). For DKIEOH we found 1.294 (± 0.018, 2σ) at 296 K, consistent with earlier observations. The carbon kinetic isotope effect in the reaction with O(1D) 13CKIEO(1D), was determined to be 1.013, whereas the deuterium kinetic isotope effect is given by DKIEO(1D) = 1.06. Both values are approximately independent of temperature between 223 and 295 K. The room temperature fractionation effect 1000(KIE-1) in the reaction of O(1D) with 12CH4 versus CH4 is thus ≈ 13‰, which is an order of magnitude greater than the previous value of 1‰. In combination with recent results from our laboratory on 13CKIE and DKIE for the reaction of CH4 with Cl, these new measurements were used to simulate the effective kinetic isotope effect for the stratosphere with a two-dimensional, time dependent chemical transport model. The model results show reasonable agreement with field observations of the 13CH4/12CH4 ratio in the lowermost stratosphere, and also reproduce the observed CH3D/CH4 ratio.

Coupled chemistry climate model simulations of the solar cycle in ozone and temperature

The 11-year solar cycles in ozone and temperature are examined using newsimulations of coupled chemistry climate models. The results show a secondary maximumin stratospheric tropical ozone, in agreement with satellite observations and in contrastwith most previously published simulations. The mean model response varies by upto about 2.5% in ozone and 0.8 K in temperature during a typical solar cycle, at the lowerend of the observed ranges of peak responses. Neither the upper atmospheric effectsof energetic particles nor the presence of the quasi biennial oscillation is necessaryto simulate the lower stratospheric response in the observed low latitude ozoneconcentration. Comparisons are also made between model simulations and observed totalcolumn ozone. As in previous studies, the model simulations agree well with observations.For those models which cover the full temporal range 1960–2005, the ozone solarsignal below 50 hPa changes substantially from the first two solar cycles to the last twosolar cycles. Further investigation suggests that this difference is due to an aliasingbetween the sea surface temperatures and the solar cycle during the first part of the period.The relationship between these results and the overall structure in the tropical solarozone response is discussed. Further understanding of solar processes requiresimprovement in the observations of the vertically varying and column integrated ozone.

Review of the formulation of present‐generation stratospheric chemistry‐climate models and associated external forcings

The goal of the Chemistry-Climate Model Validation (CCMVal) activity is to improve understanding of chemistry-climate models (CCMs) through process-oriented evaluation and to provide reliable projections of stratospheric ozone and its impact on climate. An appreciation of the details of model formulations is essential for understanding how models respond to the changing external forcings of greenhouse gases and ozone-depleting substances, and hence for understanding the ozone and climate forecasts produced by the models participating in this activity. Here we introduce and review the models used for the second round (CCMVal-2) of this intercomparison, regarding the implementation of chemical, transport, radiative, and dynamical processes in these models. In particular, we review the advantages and problems associated with approaches used to model processes of relevance to stratospheric dynamics and chemistry. Furthermore, we state the definitions of the reference simulations performed, and describe the forcing data used in these simulations. We identify some developments in chemistry-climate modeling that make models more physically based or more comprehensive, including the introduction of an interactive ocean, online photolysis, troposphere-stratosphere chemistry, and non-orographic gravity-wave deposition as linked to tropospheric convection. The relatively new developments indicate that stratospheric CCM modeling is becoming more consistent with our physically based understanding of the atmosphere.