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The control of the production of ozone-depleting substances through the Montreal Protocol means that the stratospheric ozone layer is recovering.sup.1 and that consequent increases in harmful surface ultraviolet radiation are being avoided.sup.2,3. The Montreal Protocol has co-benefits for climate change mitigation, because ozone-depleting substances are potent greenhouse gases.sup.4-7. The avoided ultraviolet radiation and climate change also have co-benefits for plants and their capacity to store carbon through photosynthesis.sup.8, but this has not previously been investigated. Here, using a modelling framework that couples ozone depletion, climate change, damage to plants by ultraviolet radiation and the carbon cycle, we explore the benefits of avoided increases in ultraviolet radiation and changes in climate on the terrestrial biosphere and its capacity as a carbon sink. Considering a range of strengths for the effect of ultraviolet radiation on plant growth.sup.8-12, we estimate that there could have been 325-690 billion tonnes less carbon held in plants and soils by the end of this century (2080-2099) without the Montreal Protocol (as compared to climate projections with controls on ozone-depleting substances). This change could have resulted in an additional 115-235 parts per million of atmospheric carbon dioxide, which might have led to additional warming of global-mean surface temperature by 0.50-1.0 degrees. Our findings suggest that the Montreal Protocol may also be helping to mitigate climate change through avoided decreases in the land carbon sink. Modelling suggests that the Montreal Protocol may be mitigating climate change by protecting the land carbon sink, as well as by protecting the ozone layer and reducing greenhouse gas emissions., Author(s): Paul J. Young [sup.1] [sup.2] [sup.3] , Anna B. Harper [sup.4] [sup.5] , Chris Huntingford [sup.6] , Nigel D. Paul [sup.1] [sup.7] , Olaf Morgenstern [sup.8] , Paul A. [...]

An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH3CCl3) (MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom-up emission inventories is the primary obstacle in choosing a MCF alternative. We illustrate that global emissions of long-lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH-SH exchange time, the emission partitioning between the two hemispheres, and the NH versus SH OH abundance ratio. Using the observed long-term trend and emissions derived from the measured hemispheric gradient, the combination of HFC-32 (CH2F2), HFC-134a (CH2FCF3, HFC-152a (CH3CHF2), and HCFC-22 (CHClF2), instead of a single gas, will be useful as a MCF alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multispecies approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF.

The most recent generation of climate models that has informed the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) is characterized by the presence of several models with larger equilibrium climate sensitivities (ECSs) and transient climate responses (TCRs) than exhibited by the previous generation. Partly as a result, AR6 did not use any direct quantifications of ECSs and TCRs based on the 4 × CO2 and 1pctCO2 simulations and relied on other evidence when assessing the Earth's actual ECS and TCR. Here I use historical observed global-mean temperature and simulations produced under the Detection and Attribution Model Intercomparison Project to constrain the ECS, TCR, and historical aerosol-related cooling. I introduce additivity criteria that disqualify 8 of the participating 16 models from consideration in multi-model averaging calculations. Based on the remaining eight models, I obtain an average adjusted ECS of 3.5 ± 0.4 K and a TCR of 1.8 ± 0.3 K (both at 68 % confidence). Both are consistent with the AR6 estimates but with substantially reduced uncertainties. Furthermore, importantly I find that the optimal cooling due to short-lived climate forcers consistent with the observed temperature record should, on average, be about 47 % ± 39 % of what these models simulate in their aerosol-only simulations, yielding a multi-model mean, global-mean, and annual-mean cooling due to near-term climate forcers for 2000–2014, relative to 1850–1899, of 0.24 ± 0.11 K (at 68 % confidence). This is consistent with but at the lower end of the very likely uncertainty range of the IPCC's AR6. There is a correlation between the models' ECSs and their aerosol-related cooling, whereby large-ECS models tend to be associated also with strong aerosol-related cooling. The results imply that a reduction in the aerosol-related cooling, along with a more moderate adjustment of the greenhouse-gas-related warming for most models, would bring the historical global-mean temperature simulated by these models into better agreement with observations. [ABSTRACT FROM AUTHOR]

This study presents an updated and homogenised ozone time series covering 34 years (1987–2020) of ozonesonde measurements at Lauder, New Zealand, and attributes vertically resolved ozone trends using a multiple linear regression (MLR) analysis and a chemistry–climate model (CCM). Homogenisation of the time series leads to a marked difference in ozone values before 1997, in which the ozone trends are predominantly negative from the surface to ∼ 30 km, ranging from ∼ - 2 % per decade to - 13 % per decade, maximising at around 12–13 km, in contrast to the uncorrected time series which shows no clear trends for this period. For the post-2000 period, ozone at Lauder shows negative trends in the stratosphere, maximising just below 20 km (∼ - 5 % per decade) despite the fact that stratospheric chlorine and bromine from ozone-depleting substances (ODSs) have both been declining since 1997. However, the ozone trends change from negative for 1987–1999 to positive in the post-2000 period in the free troposphere. The post-2000 ozone trends calculated from the ozonesonde measurements compare well with those derived from the co-located low-vertical-resolution Fourier-transform infrared spectroscopy (FTIR) ozone time series. The MLR analysis identifies that the increasing tropopause height, associated with CO2 -driven dynamical changes, is the leading factor driving the continuous negative trend in lower-stratospheric ozone at Lauder over the whole observational period, whilst the ozone-depleting substances (ODSs) only contribute to the negative ozone trend in the lower stratosphere over the pre-1999 period. Meanwhile, stratospheric temperature changes contribute significantly to the negative ozone trend above 20 km over the post-2000 period. Furthermore, the chemistry–climate model (CCM) simulations that separate the effects of individual forcings show that the predominantly negative modelled trend in ozone for the 1987–1999 period is driven not only by ODSs but also by increases in greenhouse gases (GHGs), with large but opposing impacts from methane (positive) and CO2 (negative), respectively. Over the 2000–2020 period, the model results show that the CO2 increase is the dominant driver for the negative trend in the lower stratosphere, in agreement with the MLR analysis. Although the model underestimates the observed negative ozone trend in the lower stratosphere for both periods, it clearly shows that CO2 -driven dynamical changes have played an increasingly important role in driving the lower-stratospheric ozone trends in the vicinity of Lauder. [ABSTRACT FROM AUTHOR]

In this work we analyse the impact of including a regional, high-resolution ocean model on simulated atmospheric climate in a coupled earth system model. The resolution of the regional, nested ocean model is approximately 0.2° compared to the ~1° resolution of the global ocean model within which it is embedded and this work complements previously published work on ocean circulation and marine heatwaves using this setup, referred to as the New Zealand Earth System Model, NZESM. After a brief discussion of the wider model setup, the persistent Southern Ocean warm bias in climate models and the validation data sets used, we show the effects of the altered ocean physics on air temperature, precipitation and evaporation, latent and sensible surface heat balances, westerly winds, the storm track and the effect on total cloud amount. Overall we find that the NZESM provides a better representation of regional atmospheric climate compared to its parent model - UKESM1 - although this improvement is not universal. For example, although the NZESM shows better agreement in surface air temperature within the nested ocean region, there is also some deterioration in the agreement at higher southern latitudes where the seasonal sea ice edge coincides with a transition from negative to positive correlation between air temperature and cloud amount. The lack of additional model tuning in the NZESM after the nested ocean model's inclusion largely accounts for the presence of these improvement-deterioration pairs with respect to observations. The reader is encouraged to read the paper of Behrens et al. (Behrens et al, 2020) before this one since it provides much additional information which will aid understanding. This study aims to provide a high-level reference ontology for how changing one aspect of the ocean physics in a coupled model can impact simulated atmospheric climate. [ABSTRACT FROM AUTHOR]

The most recent generation of climate models that has informed the 6th Assessment Report (AR6) of IPCC is characterized by the presence of several models with anomalously large equilibrium climate sensitivities (ECSs) relative to the previous generation. Partly as a result, AR6 did not use any direct quantifications of ECSs using 4×CO2 simulations and relied on other evidence when assessing the Earth's actual ECS. Here I use the historical observed global-mean surface air temperature and simulations produced under the Detection and AttributionModel Intercomparison Project to constrain the ECS and historical aerosol-related cooling. Based on 15 largely independent models I obtain an average adjusted ECS of 3.4±0.8 K (at 68% confidence), which is very consistent with the AR6 estimate. Furthermore, importantly I find that the optimal cooling due to anthropogenic aerosols consistent with the observed temperature record should on average be about 34±31% of what these models simulate, yielding an aerosol-related global-mean cooling for 2000-2014, relative to 1850-1899, of -0.19±0.14 K (at 68% confidence), when these models simulate on average -0.63±0.28 K. For 12 models the reduction in aerosol-related cooling equals or exceeds 50%. There is a correlation between the models' ECS and their aerosol-related cooling, whereby large-ECS models tend to be associated also with large aerosol-related cooling. The results imply that a large reduction of the aerosol-related cooling, along with a more moderate adjustment of the greenhouse-gas related warming, for most models would bring the historical global mean temperature simulated by these models into better agreement with observations. [ABSTRACT FROM AUTHOR]

This study presents an updated and homogenised ozone time series covering 34 years (1987-2020) of ozonesonde measurements at Lauder, New Zealand, and derived vertically resolved ozone trends. Over the period of 1987-1999, the ozone trends in the homogenised ozone data are predominantly negative from the surface to ~30 km, ranging from ~ -2 to -12% decade-1, maximising at around 12-13 km. These negative trends are statistically significant at 95% confidence level below 5 km and above 17 km. For the post-2000 period, ozone at Lauder shows negative trends in the stratosphere (but the trends are only statistically significant above 17 km), maximising just below 20 km (~ -5% decade-1), despite stratospheric chlorine and bromine from ozone-depleting substances (ODSs) both declining in this period. In the troposphere, the ozone trends change from negative for 1987-1999 to positive in the post-2000 period. The post-2000 ozone trends from the ozonesonde measurements compare well with those from a low-vertical resolution Fourier-transform infrared spectroscopy (FTIR) ozone time series. A multiple-linear regression analysis indicates that anthropogenic forcing plays a significant role in driving the significant negative trend in the stratospheric ozone at Lauder, in which the effect of greenhouse gas (GHG)-driven dynamical and chemical changes is reflected in the significant positive trends in tropopause height and tropospheric temperature, and significant negative trends of stratospheric temperature observed at Lauder. The interannual variation in lower stratospheric ozone is largely explained by the variation in tropopause height at Lauder, which is highly anti-correlated with stratospheric temperature and correlated with tropospheric temperature. Furthermore, the impact of ODSs and GHGs on ozone over Lauder is assessed in a chemistry-climate model using a series of single forcing simulations. The model simulations show that the predominantly negative modelled trend in ozone for the 1987-1999 period is driven not only by ODSs, but also by increases in GHGs with large but opposing impacts from methane (positive) and CO2 (negative), respectively. Over the 2000-2020 period, although the model underestimates the observed negative ozone trend in the lower stratosphere but clearly shows that CO2-driven dynamical changes have had an increasingly important role in driving ozone trends in this region. [ABSTRACT FROM AUTHOR]

This study presents an updated and homogenised ozone time series covering 34 years (1987–2020) of ozonesonde measurements at Lauder, New Zealand, and derived vertically resolved ozone trends. Over the period of 1987–1999, the ozone trends in the homogenised ozone data are predominantly negative from the surface to ∼30 km, ranging from −2 to −10 % decade−1, maximising at around 12–13 km. These negative trends are statistically significant at 95 % confidence level below 5 km and above 17 km. For the post-2000 period, ozone at Lauder shows negative trends in the stratosphere (but the trends are only statistically significant above 17 km), maximising just below 20 km (∼ −5 % decade−1), despite stratospheric chlorine and bromine from ozone-depleting substances (ODSs) both declining in this period. In the troposphere, the ozone trends change from negative for 1987–1999 to positive in the post-2000 period. The post-2000 ozone trends from the ozonesonde measurements compare well with those from a low-vertical resolution Fourier-transform infrared spectroscopy (FTIR) ozone time series. A multiple-linear regression analysis indicates that anthropogenic forcing plays a significant role in driving the significant negative trend in the stratospheric ozone at Lauder, in which the effect of greenhouse gas (GHG)-driven dynamical and chemical changes is reflected in the significant positive trends in tropopause height and tropospheric temperature, and significant negative trends of stratospheric temperature observed at Lauder. The interannual variation in lower stratospheric ozone is largely explained by the variation in tropopause height at Lauder, which is highly anti-correlated with stratospheric temperature and correlated with tropospheric temperature. Furthermore, the impact of ODSs and GHGs on ozone over Lauder is assessed in a chemistry-climate model using a series of single forcing simulations. The model simulations show that the predominantly negative modelled trend in ozone for the 1987–1999 period is driven not only by ODSs, but also by increases in GHGs with large but opposing impacts from methane (positive) and CO2 (negative), respectively. Over the 2000–2020 period, although the model underestimates the observed negative ozone trend in the lower stratosphere but clearly shows that CO2-driven dynamical changes have had an increasingly important role in driving ozone trends in this region. [ABSTRACT FROM AUTHOR]

Tropical cyclones – TCs – affecting the South Pacific region are studied using coupled atmosphere-ocean earth system models and (offline) storm tracking software which tracks the position of simulated pressure lows through time. The models used are the United Kingdom Earth System Model, version 1 – UKESM1 – and the related New Zealand Earth System Model, the NZESM. The model pair considered here differ only in their treatment of the ocean and the NZESM has a nominal resolution of 0.2° in the region surrounding New Zealand and 1° elsewhere; UKESM1 has a a uniform 1° resolution everywhere. After validating the storm tracking algorithm against the track of cyclone Giselle from 1968 and cyclone Gabrielle from 2023 we use the Saffir-Simpson scale to split the tracked systems into categories based on their severity. For systems formed in the vicinity of New Zealand (and globally) the overall number is overestimated but stronger (category 2 and 3) storms are underestimated. We also see a general decrease in the total number of storms as radiative forcing, F , increases although there is some evidence of a small increase at extreme levels of warming. In the metrics studied here we find no difference between the ensembles of UKESM1 and NZESM simulations and going forward use the UKESM1, which has larger available ensembles. The power dissipation index, PDI, gives a first order measure of TC strength and we find that the average PDI per storm increases with F by up to 26 % under a 'fossil-fuelled development' scenario. Although the physical mechanisms behind the increase in average PDI with F are relatively simple to understand, those governing the frequency of occurrence are not. In the results shown here, vertical wind shear increases with F which tends to reduce TC numbers but the effect of the tropospheric relative humidity is much less clear. The increase in the area of the tropics bounded by the 26.5° isotherm should, on its own, increase the number of TCs, in opposition to the general behaviour observed, except perhaps at extreme levels of future warming. [ABSTRACT FROM AUTHOR]