Causes of Higher Climate Sensitivity in CMIP6 Models

Equilibrium climate sensitivity, the global surface temperature response to CO urn:x-wiley:grl:media:grl60047:grl60047-math-0001 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5–4.5 K. Global climate models (GCMs), which attempt to represent all relevant physical processes, provide the most direct means of estimating climate sensitivity via CO urn:x-wiley:grl:media:grl60047:grl60047-math-0002 quadrupling experiments. Here we show that the closely related effective climate sensitivity has increased substantially in Coupled Model Intercomparison Project phase 6 (CMIP6), with values spanning 1.8–5.6 K across 27 GCMs and exceeding 4.5 K in 10 of them. This (statistically insignificant) increase is primarily due to stronger positive cloud feedbacks from decreasing extratropical low cloud coverage and albedo. Both of these are tied to the physical representation of clouds which in CMIP6 models lead to weaker responses of extratropical low cloud cover and water content to unforced variations in surface temperature. Establishing the plausibility of these higher sensitivity models is imperative given their implied societal ramifications

Causes of differences in model and satellite tropospheric warming rates

In the early twenty-first century, satellite-derived tropospheric warming trends were generally smaller than trends estimated from a large multi-model ensemble. Because observations and coupled model simulations do not have the same phasing of natural internal variability, such decadal differences in simulated and observed warming rates invariably occur. Here we analyse global-mean tropospheric temperatures from satellites and climate model simulations to examine whether warming rate differences over the satellite era can be explained by internal climate variability alone. We find that in the last two decades of the twentieth century, differences between modelled and observed tropospheric temperature trends are broadly consistent with internal variability. Over most of the early twenty-first century, however, model tropospheric warming is substantially larger than observed; warming rate differences are generally outside the range of trends arising from internal variability. The probability that multi-decadal internal variability fully explains the asymmetry between the late twentieth and early twenty-first century results is low (between zero and about 9%). It is also unlikely that this asymmetry is due to the combined effects of internal variability and a model error in climate sensitivity. We conclude that model overestimation of tropospheric warming in the early twenty-first century is partly due to systematic deficiencies in some of the post-2000 external forcings used in the model simulations

Making sense of the early-2000s warming slowdown

It has been claimed that the early-2000s global warming slowdown or hiatus, characterized by a reduced rate of global surface warming, has been overstated, lacks sound scientific basis, or is unsupported by observations. The evidence presented here contradicts these claims

Observed temperature changes in the troposphere and stratosphere from 1979 to 2018

Temperature observations of the upper-air atmosphere are now available for more than 40 years from both ground- and satellite-based observing systems. Recent years have seen substantial improvements in reducing long-standing discrepancies among datasets through major reprocessing efforts. The advent of radio occultation (RO) observations in 2001 has led to further improvements in vertically resolved temperature measurements, enabling a detailed analysis of upper-troposphere/lower-stratosphere trends. This paper presents the current state of atmospheric temperature trends from the latest available observational records. We analyze observations from merged operational satellite measurements, radiosondes, lidars, and RO, spanning a vertical range from the lower troposphere to the upper stratosphere. The focus is on assessing climate trends and on identifying the degree of consistency among the observational systems. The results show a robust cooling of the stratosphere of about 1–3 K, and a robust warming of the troposphere of about 0.6–0.8 K over the last four decades (1979– 2018). Consistent results are found between the satellite-based layer-average temperatures and vertically resolved radiosonde records. The overall latitude–altitude trend patterns are consistent between RO and radiosonde records. Significant warming of the troposphere is evident in the RO measurements available after 2001, with trends of 0.25–0.35 K per decade. Amplified warming in the tropical upper-troposphere compared to surface trends for 2002–18 is found based on RO and radiosonde records, in approximate agreement with moist adiabatic lapse rate theory. The consistency of trend results from the latest upper-air datasets will help to improve understanding of climate changes and their drivers

Spatial radiative feedbacks from internal variability using multiple regression

The sensitivity of the climate to CO2 forcing depends on spatially varying radiative feedbacks that act both locally and nonlocally. We assess whether a method employing multiple regression can be used to estimate local and nonlocal radiative feedbacks from internal variability. We test this method on millennial-length simulations performed with six coupled atmosphere-ocean general circulation models (AOGCMs). Given the spatial pattern of warming, the method does quite well at recreating the top-of-atmosphere flux response for most regions of Earth, except over the Southern Ocean where it consistently overestimates the change, leading to an overestimate of the sensitivity. For five of the six models, the method finds that local feedbacks are positive due to cloud processes, balanced by negative nonlocal shortwave cloud feedbacks associated with regions of tropical convection. For four of these models, the magnitudes of both are comparable to the Planck feedback, so that changes in the ratio between them could lead to large changes in climate sensitivity. The positive local feedback explains why observational studies that estimate spatial feedbacks using only local regressions predict an unstable climate. The method implies that sensitivity in these AOGCMs increases over time due to a reduction in the share of warming occurring in tropical convecting regions and the resulting weakening of associated shortwave cloud and longwave clear-sky feedbacks. Our results provide a step toward an observational estimate of time-varying climate sensitivity by demonstrating that many aspects of spatial feedbacks appear to be the same between internal variability and the forced response

State of the climate in 2018

In 2018, the dominant greenhouse gases released into Earth’s atmosphere—carbon dioxide, methane, and nitrous oxide—continued their increase. The annual global average carbon dioxide concentration at Earth’s surface was 407.4 ± 0.1 ppm, the highest in the modern instrumental record and in ice core records dating back 800 000 years. Combined, greenhouse gases and several halogenated gases contribute just over 3 W m−2 to radiative forcing and represent a nearly 43% increase since 1990. Carbon dioxide is responsible for about 65% of this radiative forcing. With a weak La Niña in early 2018 transitioning to a weak El Niño by the year’s end, the global surface (land and ocean) temperature was the fourth highest on record, with only 2015 through 2017 being warmer. Several European countries reported record high annual temperatures. There were also more high, and fewer low, temperature extremes than in nearly all of the 68-year extremes record. Madagascar recorded a record daily temperature of 40.5°C in Morondava in March, while South Korea set its record high of 41.0°C in August in Hongcheon. Nawabshah, Pakistan, recorded its highest temperature of 50.2°C, which may be a new daily world record for April. Globally, the annual lower troposphere temperature was third to seventh highest, depending on the dataset analyzed. The lower stratospheric temperature was approximately fifth lowest. The 2018 Arctic land surface temperature was 1.2°C above the 1981–2010 average, tying for third highest in the 118-year record, following 2016 and 2017. June’s Arctic snow cover extent was almost half of what it was 35 years ago. Across Greenland, however, regional summer temperatures were generally below or near average. Additionally, a satellite survey of 47 glaciers in Greenland indicated a net increase in area for the first time since records began in 1999. Increasing permafrost temperatures were reported at most observation sites in the Arctic, with the overall increase of 0.1°–0.2°C between 2017 and 2018 being comparable to the highest rate of warming ever observed in the region. On 17 March, Arctic sea ice extent marked the second smallest annual maximum in the 38-year record, larger than only 2017. The minimum extent in 2018 was reached on 19 September and again on 23 September, tying 2008 and 2010 for the sixth lowest extent on record. The 23 September date tied 1997 as the latest sea ice minimum date on record. First-year ice now dominates the ice cover, comprising 77% of the March 2018 ice pack compared to 55% during the 1980s. Because thinner, younger ice is more vulnerable to melting out in summer, this shift in sea ice age has contributed to the decreasing trend in minimum ice extent. Regionally, Bering Sea ice extent was at record lows for almost the entire 2017/18 ice season. For the Antarctic continent as a whole, 2018 was warmer than average. On the highest points of the Antarctic Plateau, the automatic weather station Relay (74°S) broke or tied six monthly temperature records throughout the year, with August breaking its record by nearly 8°C. However, cool conditions in the western Bellingshausen Sea and Amundsen Sea sector contributed to a low melt season overall for 2017/18. High SSTs contributed to low summer sea ice extent in the Ross and Weddell Seas in 2018, underpinning the second lowest Antarctic summer minimum sea ice extent on record. Despite conducive conditions for its formation, the ozone hole at its maximum extent in September was near the 2000–18 mean, likely due to an ongoing slow decline in stratospheric chlorine monoxide concentration. Across the oceans, globally averaged SST decreased slightly since the record El Niño year of 2016 but was still far above the climatological mean. On average, SST is increasing at a rate of 0.10° ± 0.01°C decade−1 since 1950. The warming appeared largest in the tropical Indian Ocean and smallest in the North Pacific. The deeper ocean continues to warm year after year. For the seventh consecutive year, global annual mean sea level became the highest in the 26-year record, rising to 81 mm above the 1993 average. As anticipated in a warming climate, the hydrological cycle over the ocean is accelerating: dry regions are becoming drier and wet regions rainier. Closer to the equator, 95 named tropical storms were observed during 2018, well above the 1981–2010 average of 82. Eleven tropical cyclones reached Saffir–Simpson scale Category 5 intensity. North Atlantic Major Hurricane Michael’s landfall intensity of 140 kt was the fourth strongest for any continental U.S. hurricane landfall in the 168-year record. Michael caused more than 30 fatalities and $25 billion (U.S. dollars) in damages. In the western North Pacific, Super Typhoon Mangkhut led to 160 fatalities and$6 billion (U.S. dollars) in damages across the Philippines, Hong Kong, Macau, mainland China, Guam, and the Northern Mariana Islands. Tropical Storm Son-Tinh was responsible for 170 fatalities in Vietnam and Laos. Nearly all the islands of Micronesia experienced at least moderate impacts from various tropical cyclones. Across land, many areas around the globe received copious precipitation, notable at different time scales. Rodrigues and Réunion Island near southern Africa each reported their third wettest year on record. In Hawaii, 1262 mm precipitation at Waipā Gardens (Kauai) on 14–15 April set a new U.S. record for 24-h precipitation. In Brazil, the city of Belo Horizonte received nearly 75 mm of rain in just 20 minutes, nearly half its monthly average. Globally, fire activity during 2018 was the lowest since the start of the record in 1997, with a combined burned area of about 500 million hectares. This reinforced the long-term downward trend in fire emissions driven by changes in land use in frequently burning savannas. However, wildfires burned 3.5 million hectares across the United States, well above the 2000–10 average of 2.7 million hectares. Combined, U.S. wildfire damages for the 2017 and 2018 wildfire seasons exceeded $40 billion (U.S. dollars)

Reconciling tropospheric temperature trends from the microwave sounding unit

Thesis (Master's)--University of Washington, 2012The University of Alabama at Huntsville (UAH), Remote Sensing Systems (RSS), and the National Oceanic and Atmospheric Administration (NOAA) have constructed long-term temperature records for deep atmospheric layers using satellite microwave sounding unit (MSU) and advanced microwave sounding unit (AMSU) observations. However, these groups disagree on the magnitude of global temperature trends since 1979, including the trend for the mid-tropospheric layer (TMT). This study evaluates the selection of the MSU TMT warm target factor for the NOAA-9 satellite using five homogenized radiosonde products as references. The analysis reveals that the UAH TMT product has a positive bias of 0.051 ± 0.031 in the warm target factor that artificially reduces the global TMT trend by 0.042 K decade−1 for 1979 − 2009. Accounting for this bias, we estimate that the global UAH TMT trend should increase from 0.038 K decade−1 to 0.080 K decade−1, effectively eliminating the trend difference between UAH and RSS and decreasing the trend difference between UAH and NOAA by 47%. This warm target factor bias directly affects the UAH lower tropospheric (TLT) product and tropospheric temperature trends derived from a combination of TMT and lower stratospheric (TLS) channels

On the structure of atmospheric warming in models and observations: Implications for the lapse rate feedback

Thesis (Ph.D.)--University of Washington, 2016-12This dissertation investigates the structure of atmospheric warming in observations and general circulation models (GCMs). Theory and GCMs suggest that warming is amplified in the tropical upper troposphere relative to the lower troposphere and the surface -- a phenomenon known as vertical amplification. We assess model and observational agreement using several amplification metrics derived from the satellite-borne microwave sounding unit (MSU) atmospheric temperature trends. An important correction to the satellite microwave record is the removal of temperature drifts caused by changes in diurnal sampling. This correction the principal source of uncertainty in microwave temperature datasets. Furthermore, in three existing datasets, the ratio of tropical warming between the upper troposphere (T24 channel) and the surface (dT24/dTs ~ 0.6 -- 1.3) is lower than that of GCMs (~1.4 -- 1.6). To better understand these issues, we produced an alternate MSU dataset with an improved diurnal correction. We show that existing MSU datasets likely underestimate tropical mid-tropospheric temperature trends. Subsequent improvements to MSU datasets using similar diurnal correction techniques leads to amplification ratios (between T24 and the surface) that are in accord with models. Another measure of tropical tropospheric amplification is the relative warming between the upper troposphere (T24) and the lower-middle troposphere (TLT). We show that most GCMs have excessive T24/TLT amplification compared to satellite microwave observations, even when models are forced with prescribed sea-surface temperatures (SSTs). A number of possible reasons for this discrepancy are assessed. Observational uncertainty in the satellite microwave record is substantial and, when taken into account, many models agree with observations within the observational uncertainty range, though about half of the model ensemble members considered still have significant discrepancies compared to observations. Our findings indicate that the prescribed ozone and stratospheric aerosol forcings do not effect T24/TLT amplification in models. On the other hand, model parameterizations for convection and microphysics and, to a lesser degree, uncertainty in the prescribed SST dataset can influence model amplification behavior and bring models into closer accord with observations. In all, significant T24/TLT discrepancies between models and observations remain, but may be reduced with improved model parameterizations. An underlying motivation for understanding the structure of atmospheric warming is that it is responsible for a large negative lapse rate feedback in future climate simulations. To understand factors that control the global lapse rate feedback across models, we use principal component analysis to find the modes of variability that best explain variance in the local lapse rate feedback. We find that models exhibit marked variability in the lapse rate feedback in the southern hemisphere extratropics. This mode is strongly correlated with the global average lapse rate feedback and is largely a function of the competing influence of tropical and Antarctic surface warming. We show that muted southern ocean sea surface warming and the non-local influence of tropical surface warming contributes to a highly variable lapse rate feedback in the sub-Antarctic across models. This behavior is dissimilar to northern hemisphere high latitudes, which are characterized by strong Arctic amplification and a relatively uniform local lapse rate feedback across GCMs. Climatological Antarctic sea ice extent influences Antarctic warming and, as a result, influences both the meridional profile of warming in the southern hemisphere and the global lapse rate feedback

Using particle induced x-ray analysis and plasma mass spectrometry to Investigate mercury emissions from anthropogenic sources

Inductively Coupled Plasma Mass Spectrometry, ICP-MS, were used to investigate mercury pollution in Norway Spruce tree bark (Picea abies) located downwind from the Huntley coal-fired power plant in North Tonawanda, New York. We predicted that mercury concentration, C, would fall inversely with distance, r, and experimentally determined that after a 5.77 km – 9.47 km deposition delay, mercury concentration fell as C ∝r-1.48 with an r2 value of 0.96. PIXE was useful in quickly and non-destructively (experiment could be done repeatedly) determining order of magnitude concentrations for heavy metals with concentrations less than one part-per-million. ICP-MS requires a more involved sample preparation method, but was useful in very precisely determining mercury concentrations. After comparing PIXE to ICP-MS, we found that the two methods were in agreement, although with varying precision. For the comparison sample, we found a PIXE value of 100 ± 70 μg kg-1 compared to 43 ± 1 μg kg-1 for ICPMS (mass of mercury per mass of bark). Using the experimental concentration – distance relationship, we found that the Huntley Power Plant had a sphere of influence (mercury concentration above background levels) of 100 – 200 km. Department of Physics and Astronomy, June 2008

Effects of Starvation on Amino Nitrogen in the Blood of Oriental Beetle (Anomala orientalis Waterhouse) Larvae as Determined by Paper Partition Chromatography

Abstract not availabl