Guang Zeng

Hydroxyl (OH) is the atmosphere’s main oxidant removing most pollutants including methane. Its short lifetime prevents large-scale direct observational quantification. Abundances inferred using anthropogenic trace gas measurements and models yield conflicting trend estimates. By contrast, radiocarbon monoxide (¹⁴CO), produced naturally by cosmic rays and almost exclusively removed by OH, is a tracer with a well-understood source. Here we show that Southern-Hemisphere ¹⁴CO measurements indicate increasing OH. New Zealand ¹⁴CO data exhibit an annual-mean decrease of 12 ± 2% since 1997, whereas Antarctic measurements show a December-January decrease of 43 ± 24%. Both imply similar OH increases, corroborating our own and other model results suggesting that OH has been globally increasing during recent decades. Model sensitivity simulations illustrate the roles of methane, nitrogen oxides, stratospheric ozone depletion, and global warming driving these trends. They have substantial implications for the budgets of pollutants removed by OH, and especially imply larger than documented methane emission increases.

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.

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.

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.

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.

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.

In the Arctic stratosphere, the combination of chemical ozone depletion by halogenated ozone-depleting substances (hODSs) and dynamic fluctuations can lead to severe ozone minima. These Arctic ozone minima are of great societal concern due to their health and climate impacts. Owing to the success of the Montreal Protocol, hODSs in the stratosphere are gradually declining, resulting in a recovery of the ozone layer. On the other hand, continued greenhouse gas (GHG) emissions cool the stratosphere, possibly enhancing the formation of polar stratospheric clouds (PSCs) and, thus, enabling more efficient chemical ozone destruction. Other processes, such as the acceleration of the Brewer–Dobson circulation, also affect stratospheric temperatures, further complicating the picture. Therefore, it is currently unclear whether major Arctic ozone minima will still occur at the end of the 21st century despite decreasing hODSs. We have examined this question for different emission pathways using simulations conducted within the Chemistry-Climate Model Initiative (CCMI-1 and CCMI-2022) and found large differences in the models' ability to simulate the magnitude of ozone minima in the present-day climate. Models with a generally too-cold polar stratosphere (cold bias) produce pronounced ozone minima under present-day climate conditions because they simulate more PSCs and, thus, high concentrations of active chlorine species (ClO_x). These models predict the largest decrease in ozone minima in the future. Conversely, models with a warm polar stratosphere (warm bias) have the smallest sensitivity of ozone minima to future changes in hODS and GHG concentrations. As a result, the scatter among models in terms of the magnitude of Arctic spring ozone minima will decrease in the future. Overall, these results suggest that Arctic ozone minima will become weaker over the next decades, largely due to the decline in hODS abundances. We note that none of the models analysed here project a notable increase of ozone minima in the future. Stratospheric cooling caused by increasing GHG concentrations is expected to play a secondary role as its effect in the Arctic stratosphere is weakened by opposing radiative and dynamical mechanisms.

In the Arctic stratosphere, the combination of chemical ozone depletion by halogenated ozone-depleting substances (hODSs) and dynamic fluctuations can lead to severe ozone minima. These Arctic ozone minima are of great societal concern due to their health and climate impacts. Owing to the success of the Montreal Protocol, hODSs in the stratosphere are gradually declining, resulting in a recovery of the ozone layer. On the other hand, continued greenhouse gas (GHG) emissions cool the stratosphere, possibly enhancing the formation of polar stratospheric clouds (PSCs) and, thus, enabling more efficient chemical ozone destruction. Other processes, such as the acceleration of the Brewer-Dobson circulation, also affect stratospheric temperatures, further complicating the picture. Therefore, it is currently unclear whether major Arctic ozone minima will still occur at the end of the 21st century despite decreasing hODSs. We have examined this question for different emission pathways using simulations conducted within the Chemistry-Climate Model Initiative (CCMI-1 and CCMI-2022) and find large differences in the models' ability to simulate the magnitude of ozone minima in the present-day climate. Models with a generally too cold polar stratosphere ("cold bias") produce pronounced ozone minima under present-day climate conditions, because they simulate more PSCs and, thus, high concentrations of active chlorine species (ClOx). These models predict the largest decrease in ozone minima in the future. Conversely, models with a warm polar stratosphere ("warm bias") have the smallest sensitivity of ozone minima to future changes in hODS and GHG concentrations. As a result, the scatter among models in the magnitude of Arctic spring ozone minima will decrease in the future. Overall, these results suggest that Arctic ozone minima will become weaker over the next decades, largely due to the decline in hODS abundances. We note that none of the models analysed here project a notable increase of ozone minima in the future. Stratospheric cooling caused by increasing GHG concentrations is expected to play a secondary role, as its effect in the Arctic stratosphere is weakened by opposing radiative and dynamical mechanisms.

In the Arctic stratosphere, the combination of chemical ozone depletion by halogenated ozone-depleting substances (hODSs) and dynamic fluctuations can lead to severe ozone minima. These Arctic ozone minima are of great societal concern due to their health and climate impacts. Owing to the success of the Montreal Protocol, hODSs in the stratosphere are gradually declining, resulting in a recovery of the ozone layer. On the other hand, continued greenhouse gas (GHG) emissions cool the stratosphere, possibly enhancing the formation of polar stratospheric clouds (PSCs) and, thus, enabling more efficient chemical ozone destruction. Other processes, such as the acceleration of the Brewer-Dobson circulation, also affect stratospheric temperatures, further complicating the picture. Therefore, it is currently unclear whether major Arctic ozone minima will still occur at the end of the 21st century despite decreasing hODSs. We have examined this question for different emission pathways using simulations conducted within the Chemistry-Climate Model Initiative (CCMI-1 and CCMI-2022) and find large differences in the models' ability to simulate the magnitude of ozone minima in the present-day climate. Models with a generally too cold polar stratosphere ("cold bias") produce pronounced ozone minima under present-day climate conditions, because they simulate more PSCs and, thus, high concentrations of active chlorine species (ClOx). These models predict the largest decrease in ozone minima in the future. Conversely, models with a warm polar stratosphere ("warm bias") have the smallest sensitivity of ozone minima to future changes in hODS and GHG concentrations. As a result, the scatter among models in the magnitude of Arctic spring ozone minima will decrease in the future. Overall, these results suggest that Arctic ozone minima will become weaker over the next decades, largely due to the decline in hODS abundances. We note that none of the models analysed here project a notable increase of ozone minima in the future. Stratospheric cooling caused by increasing GHG concentrations is expected to play a secondary role, as its effect in the Arctic stratosphere is weakened by opposing radiative and dynamical mechanisms.

Water vapor plays an important role in many aspects of the climate system, by affecting radiation, cloud formation, atmospheric chemistry and dynamics. Even the low stratospheric water vapor content provides an important climate feedback, but current climate models show a substantial moist bias in the lowermost stratosphere. Here we report crucial sensitivity of the atmospheric circulation in the stratosphere and troposphere to the abundance of water vapor in the lowermost stratosphere. We show from a mechanistic climate model experiment and inter-model variability that lowermost stratospheric water vapor decreases local temperatures, and thereby causes an upward and poleward shift of subtropical jets, a strengthening of the stratospheric circulation, a poleward shift of the tropospheric eddy-driven jet and regional climate impacts. The mechanistic model experiment in combination with atmospheric observations further shows that the prevailing moist bias in current models is likely caused by the transport scheme, and can be alleviated by employing a less diffusive Lagrangian scheme. The related effects on atmospheric circulation are of similar magnitude as climate change effects. Hence, lowermost stratospheric water vapor exerts a first order effect on atmospheric circulation and improving its representation in models offers promising prospects for future research.