For the current generation of earth system models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), the range of equilibrium climate sensitivity (ECS, a hypothetical value of global warming at equilibrium for a doubling of CO2) is 18°C to 56°C, the largest of any generation of models dating to the 1990s Meanwhile, the range of transient climate response (TCR, the surface temperature warming around the time of CO2 doubling in a 1% per year CO2 increase simulation) for the CMIP6 models of 17°C (13°C to 30°C) is only slightly larger than for the CMIP3 and CMIP5 models Here we review and synthesize the latest developments in ECS and TCR values in CMIP, compile possible reasons for the current values as supplied by the modeling groups, and highlight future directions Cloud feedbacks and cloud-aerosol interactions are the most likely contributors to the high values and increased range of ECS in CMIP6

Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models.

Abstract In recent decades, the warming in the Arctic has been much faster than in the rest of the world, a phenomenon known as Arctic amplification. Numerous studies report that the Arctic is warming either twice, more than twice, or even three times as fast as the globe on average. Here we show, by using several observational datasets which cover the Arctic region, that during the last 43 years the Arctic has been warming nearly four times faster than the globe, which is a higher ratio than generally reported in literature. We compared the observed Arctic amplification ratio with the ratio simulated by state-of-the-art climate models, and found that the observed four-fold warming ratio over 1979–2021 is an extremely rare occasion in the climate model simulations. The observed and simulated amplification ratios are more consistent with each other if calculated over a longer period; however the comparison is obscured by observational uncertainties before 1979. Our results indicate that the recent four-fold Arctic warming ratio is either an extremely unlikely event, or the climate models systematically tend to underestimate the amplification. 

/pdf/the-arctic-has-warmed-nearly-four-times-faster-than-the-2f7lp97b.pdf

The Arctic has warmed nearly four times faster than the globe since 1979

. Anthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation,
reductions in near-surface nutrients, and changes to primary production, all of which are expected
to affect marine ecosystems. Here we assess projections of these drivers of environmental change
over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model
Intercomparison Project Phase 6 (CMIP6) that were forced under the CMIP6 Shared Socioeconomic
Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced
under the Representative Concentration Pathways (RCPs). A total of 10 CMIP5 and 13 CMIP6 models are used in
the two multi-model ensembles. Under the high-emission scenario SSP5-8.5, the multi-model global
mean change (2080–2099 mean values relative to 1870–1899) ± the inter-model SD in sea
surface temperature, surface pH, subsurface (100–600  m ) oxygen concentration, euphotic
(0–100  m ) nitrate concentration, and depth-integrated primary production is
 + 3.47 ± 0.78 ∘C , - 0.44 ± 0.005 , - 13.27 ± 5.28 ,
 - 1.06 ± 0.45 mmol m−3 and - 2.99 ± 9.11  %, respectively. Under the
low-emission, high-mitigation scenario SSP1-2.6, the corresponding global changes are
 + 1.42 ± 0.32 ∘C , - 0.16 ± 0.002 , - 6.36 ± 2.92 ,
 - 0.52 ± 0.23 mmol m−3 , and - 0.56 ± 4.12  %. Projected exposure of the marine
ecosystem to these drivers of ocean change depends largely on the extent of future emissions,
consistent with previous studies. The ESMs in CMIP6 generally project greater warming,
acidification, deoxygenation, and nitrate reductions but lesser primary production declines than
those from CMIP5 under comparable radiative forcing. The increased projected ocean warming results
from a general increase in the climate sensitivity of CMIP6 models relative to those of
CMIP5. This enhanced warming increases upper-ocean stratification in CMIP6 projections, which
contributes to greater reductions in upper-ocean nitrate and subsurface oxygen ventilation. The
greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher
associated atmospheric CO2 concentrations than their RCP analogues for the same
radiative forcing. We find no consistent reduction in inter-model uncertainties, and even an
increase in net primary
production inter-model uncertainties in CMIP6, as compared to CMIP5.

/pdf/twenty-first-century-ocean-warming-acidification-2b9m6j9g04.pdf

Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections

The latest Coupled Model Intercomparison Project phase 6 (CMIP6) dataset was analyzed to examine the projected changes in temperature and precipitation over six South Asian countries during the twenty-first century. The CMIP6 model simulations reveal biases in annual mean temperature and precipitation over South Asia in the present climate. In the historical period, the median of the CMIP6 model ensemble systematically underestimates the annual mean temperature for all the South Asian countries, while a mixed behavior is shown in the case of precipitation. In the future climate, the CMIP6 models display higher sensitivity to greenhouse gas emissions over South Asia compared with the CMIP5 models. The multimodel ensemble from 27 CMIP6 models projects a continuous increase in the annual mean temperature over South Asia during the twenty-first century under three future scenarios. The projected temperature shows a large increase (over 6 °C under SSP5-8.5 scenario) over the northwestern parts of South Asia, comprising the complex Karakorum and Himalayan mountain ranges. Any large increase in the mean temperature over this region will most likely result in a faster rate of glacier melting. By the end of the twenty-first century, the annual mean temperature (uncertainty range) over South Asia is projected to increase by 1.2 (0.7–2.1) °C, 2.1 (1.5–3.3) °C, and 4.3 (3.2–6.6) °C under the SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios, respectively, relative to the present (1995–2014) climate. The warming over South Asia is also continuous on the seasonal time scale. The CMIP6 models projected higher warming in the winter season than in the summer over South Asia, which if verified will have repercussions for snow/ice accumulations as well as winter cropping patterns. The annual mean precipitation is also projected to increase over South Asia during the twenty-first century under all scenarios. The rate of change in the projected annual mean precipitation varies considerably between the South Asian countries. By the end of the twenty-first century, the country-averaged annual mean precipitation (uncertainty range) is projected to increase by 17.1 (2.2–49.1)% in Bangladesh, 18.9 (−4.9 to 72)% in Bhutan, 27.3 (5.3–160.5)% in India, 19.5 (−5.9 to 95.6)% in Nepal, 26.4 (6.4–159.7)% in Pakistan, and 25.1 (−8.5 to 61.0)% in Sri Lanka under the SSP5-8.5 scenario. The seasonal precipitation projections also shows large variability. The projected winter precipitation reveals a robust increase over the western Himalayas, with a corresponding decrease over the eastern Himalayas. On the other hand, the summer precipitation shows a robust increase over most of the South Asia region, with the largest increase over the arid region of southern Pakistan and adjacent areas of India, under the high-emission scenario. The results presented in this study give detailed insights into CMIP6 model performance over the South Asia region, which could be extended further to develop adaptation strategies, and may act as a guideline document for climate change related policymaking in the region.

/pdf/projections-of-precipitation-and-temperature-over-the-south-27t03b9ls3.pdf

Projections of Precipitation and Temperature over the South Asian Countries in CMIP6

. Results from the fully and biogeochemically coupled simulations in which
 CO2 increases at a rate of 1 % yr −1 
(1pctCO2) from its
preindustrial value are analyzed to quantify the magnitude of
carbon–concentration and carbon–climate feedback parameters which measure
the response of ocean and terrestrial carbon pools to changes in atmospheric
 CO2 concentration and the resulting change in global climate,
respectively. The results are based on 11 comprehensive Earth system
models from the most recent (sixth) Coupled Model Intercomparison Project
(CMIP6) and compared with eight models from the fifth CMIP (CMIP5). The
strength of the carbon–concentration feedback is of comparable magnitudes
over land (mean  ±  standard deviation  =  0.97  ±  0.40 PgC ppm −1 ) and ocean (0.79  ±  0.07 PgC ppm −1 ), while the
carbon–climate feedback over land ( −45.1 ±  50.6 PgC  ∘ C −1 ) is about 3 times larger than over ocean ( −17.2 ±  5.0 PgC  ∘ C −1 ). The strength of both feedbacks is an order of
magnitude more uncertain over land than over ocean as has been seen in
existing studies. These values and their spread from 11 CMIP6 models
have not changed significantly compared to CMIP5 models. The absolute values
of feedback parameters are lower for land with models that include a
representation of nitrogen cycle. The transient climate response to
cumulative emissions (TCRE) from the 11 CMIP6 models considered here is
1.77  ±  0.37  ∘ C EgC −1 and is similar to that found in
CMIP5 models (1.63  ±  0.48  ∘ C EgC −1 ) but with somewhat
reduced model spread. The expressions for feedback parameters based on the
fully and biogeochemically coupled configurations of the 1pctCO2 simulation
are simplified when the small temperature change in the
biogeochemically coupled simulation is ignored. Decomposition of the terms
of these simplified expressions for the feedback parameters is used to gain
insight into the reasons for differing responses among ocean and land carbon
cycle models.

/pdf/carbon-concentration-and-carbon-climate-feedbacks-in-cmip6-2423t393k5.pdf

Carbon-concentration and carbon-climate feedbacks in CMIP6 models and their comparison to CMIP5 models

. The Canadian Earth System Model version 5 (CanESM5) is a global
model developed to simulate historical climate change and variability, to
make centennial-scale projections of future climate, and to produce
initialized seasonal and decadal predictions. This paper describes the model
components and their coupling, as well as various aspects of model
development, including tuning, optimization, and a reproducibility strategy.
We also document the stability of the model using a long control simulation,
quantify the model's ability to reproduce large-scale features of the
historical climate, and evaluate the response of the model to external
forcing. CanESM5 is comprised of three-dimensional atmosphere (T63 spectral
resolution equivalent roughly to 2.8 ∘ ) and ocean (nominally 1 ∘ ) general
circulation models, a sea-ice model, a land surface scheme, and explicit
land and ocean carbon cycle models. The model features relatively coarse
resolution and high throughput, which facilitates the production of large
ensembles. CanESM5 has a notably higher equilibrium climate sensitivity
(5.6 K) than its predecessor, CanESM2 (3.7 K), which we briefly discuss, along
with simulated changes over the historical period. CanESM5 simulations
contribute to the Coupled Model Intercomparison Project phase 6 (CMIP6)
and will be employed for climate science and service applications in Canada.

Fouad Majaess

Papers

The Canadian Earth System Model version 5 (CanESM5.0.3)

CMIP6.ScenarioMIP.CCCma.CanESM5

CMIP6.ScenarioMIP.CCCma.CanESM5.ssp585

CMIP6.ScenarioMIP.CCCma.CanESM5.ssp245