The term "tipping point" commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term "tipping element" to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.

/pdf/tipping-elements-in-the-earth-s-climate-system-3h0whqwzeb.pdf

Tipping elements in the Earth's climate system

This chapter assesses long-term projections of climate change for the end of the 21st century and beyond, where the forced signal depends on the scenario and is typically larger than the internal variability of the climate system. Changes are expressed with respect to a baseline period of 1986-2005, unless otherwise stated.

Chapter 12 - Long-term climate change: Projections, commitments and irreversibility

Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate – the “global carbon budget” – is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) and terrestrial CO2 sink (SLAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the last decade available (2010–2019), EFOS was 9.6 ± 0.5 GtC yr−1 excluding the cement carbonation sink (9.4 ± 0.5 GtC yr−1 when the cement carbonation sink is included), and ELUC was 1.6 ± 0.7 GtC yr−1. For the same decade, GATM was 5.1 ± 0.02 GtC yr−1 (2.4 ± 0.01 ppm yr−1), SOCEAN 2.5 ±  0.6 GtC yr−1, and SLAND 3.4 ± 0.9 GtC yr−1, with a budget imbalance BIM of −0.1 GtC yr−1 indicating a near balance between estimated sources and sinks over the last decade. For the year 2019 alone, the growth in EFOS was only about 0.1 % with fossil emissions increasing to 9.9 ± 0.5 GtC yr−1 excluding the cement carbonation sink (9.7 ± 0.5 GtC yr−1 when cement carbonation sink is included), and ELUC was 1.8 ± 0.7 GtC yr−1, for total anthropogenic CO2 emissions of 11.5 ± 0.9 GtC yr−1 (42.2 ± 3.3 GtCO2). Also for 2019, GATM was 5.4 ± 0.2 GtC yr−1 (2.5 ± 0.1 ppm yr−1), SOCEAN was 2.6 ± 0.6 GtC yr−1, and SLAND was 3.1 ± 1.2 GtC yr−1, with a BIM of 0.3 GtC. The global atmospheric CO2 concentration reached 409.85 ± 0.1 ppm averaged over 2019. Preliminary data for 2020, accounting for the COVID-19-induced changes in emissions, suggest a decrease in EFOS relative to 2019 of about −7 % (median estimate) based on individual estimates from four studies of −6 %, −7 %, −7 % (−3 % to −11 %), and −13 %. Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2019, but discrepancies of up to 1 GtC yr−1 persist for the representation of semi-decadal variability in CO2 fluxes. Comparison of estimates from diverse approaches and observations shows (1) no consensus in the mean and trend in land-use change emissions over the last decade, (2) a persistent low agreement between the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) an apparent discrepancy between the different methods for the ocean sink outside the tropics, particularly in the Southern Ocean. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding of the global carbon cycle compared with previous publications of this data set (Friedlingstein et al., 2019; Le Quere et al., 2018b, a, 2016, 2015b, a, 2014, 2013). The data presented in this work are available at https://doi.org/10.18160/gcp-2020 (Friedlingstein et al., 2020).

/pdf/global-carbon-budget-2020-2dj69h8ged.pdf

Global Carbon Budget 2020

[1] The new Max-Planck-Institute Earth System Model (MPI-ESM) is used in the Coupled Model Intercomparison Project phase 5 (CMIP5) in a series of climate change experiments for either idealized CO2-only forcing or forcings based on observations and the Representative Concentration Pathway (RCP) scenarios. The paper gives an overview of the model configurations, experiments related forcings, and initialization procedures and presents results for the simulated changes in climate and carbon cycle. It is found that the climate feedback depends on the global warming and possibly the forcing history. The global warming from climatological 1850 conditions to 2080–2100 ranges from 1.5°C under the RCP2.6 scenario to 4.4°C under the RCP8.5 scenario. Over this range, the patterns of temperature and precipitation change are nearly independent of the global warming. The model shows a tendency to reduce the ocean heat uptake efficiency toward a warmer climate, and hence acceleration in warming in the later years. The precipitation sensitivity can be as high as 2.5% K−1 if the CO2 concentration is constant, or as small as 1.6% K−1, if the CO2 concentration is increasing. The oceanic uptake of anthropogenic carbon increases over time in all scenarios, being smallest in the experiment forced by RCP2.6 and largest in that for RCP8.5. The land also serves as a net carbon sink in all scenarios, predominantly in boreal regions. The strong tropical carbon sources found in the RCP2.6 and RCP8.5 experiments are almost absent in the RCP4.5 experiment, which can be explained by reforestation in the RCP4.5 scenario.

Climate and carbon cycle changes from 1850 to 2100 in MPI‐ESM simulations for the Coupled Model Intercomparison Project phase 5

. Accurate assessment of anthropogenic carbon dioxide ( CO2 ) emissions and
their redistribution among the atmosphere, ocean, and terrestrial biosphere
– the “global carbon budget” – is important to better understand the
global carbon cycle, support the development of climate policies, and
project future climate change. Here we describe data sets and methodology to
quantify the five major components of the global carbon budget and their
uncertainties. Fossil CO2 emissions ( EFF ) are based on energy
statistics and cement production data, while emissions from land use change
( ELUC ), mainly deforestation, are based on land use and land use change
data and bookkeeping models. Atmospheric CO2 concentration is measured
directly and its growth rate ( GATM ) is computed from the annual changes
in concentration. The ocean CO2 sink ( SOCEAN ) and terrestrial
 CO2 sink ( SLAND ) are estimated with global process models
constrained by observations. The resulting carbon budget imbalance
( BIM ), the difference between the estimated total emissions and the
estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a
measure of imperfect data and understanding of the contemporary carbon
cycle. All uncertainties are reported as ±1σ . For the last
decade available (2009–2018), EFF was 9.5±0.5  GtC yr −1 ,
 ELUC 1.5±0.7  GtC yr −1 , GATM 4.9±0.02  GtC yr −1 ( 2.3±0.01  ppm yr −1 ), SOCEAN 2.5±0.6  GtC yr −1 , and SLAND 3.2±0.6  GtC yr −1 , with a budget
imbalance BIM of 0.4 GtC yr −1 indicating overestimated emissions
and/or underestimated sinks. For the year 2018 alone, the growth in EFF was
about 2.1 % and fossil emissions increased to 10.0±0.5  GtC yr −1 , reaching 10 GtC yr −1 for the first time in history,
 ELUC was 1.5±0.7  GtC yr −1 , for total anthropogenic
 CO2 emissions of 11.5±0.9  GtC yr −1 ( 42.5±3.3 GtCO2 ). Also for 2018, GATM was 5.1±0.2  GtC yr −1 ( 2.4±0.1  ppm yr −1 ), SOCEAN was 2.6±0.6  GtC yr −1 , and SLAND was 3.5±0.7  GtC yr −1 , with a BIM of 0.3 GtC. The global atmospheric CO2 concentration reached 407.38±0.1  ppm averaged over 2018. For 2019, preliminary data for the first 6–10 months indicate a reduced growth in EFF of +0.6  % (range of
 −0.2  % to 1.5 %) based on national emissions projections for China, the
USA, the EU, and India and projections of gross domestic product corrected
for recent changes in the carbon intensity of the economy for the rest of
the world. Overall, the mean and trend in the five components of the global
carbon budget are consistently estimated over the period 1959–2018, but
discrepancies of up to 1 GtC yr −1 persist for the representation of
semi-decadal variability in CO2 fluxes. A detailed comparison among
individual estimates and the introduction of a broad range of observations
shows (1) no consensus in the mean and trend in land use change emissions
over the last decade, (2) a persistent low agreement between the different
methods on the magnitude of the land CO2 flux in the northern
extra-tropics, and (3) an apparent underestimation of the CO2 
variability by ocean models outside the tropics. This living data update
documents changes in the methods and data sets used in this new global
carbon budget and the progress in understanding of the global carbon cycle
compared with previous publications of this data set (Le Quere et
al., 2018a, b, 2016, 2015a, b, 2014, 2013). The data generated by
this work are available at https://doi.org/10.18160/gcp-2019 (Friedlingstein
et al., 2019).

/pdf/global-carbon-budget-2019-r2v2pjpie5.pdf

Global Carbon Budget 2019

CMIP6.CMIP.MOHC.HadGEM3-GC31-LL.historical

The meridional overturning circulation (MOC) is part of a global ocean circulation that redistributes heat from
Equatorial to Polar regions. In the Atlantic, the MOC (AMOC) carries heat northward (the Atlantic Heat Conveyor)
which is released to the atmosphere and maintains UK temperatures between 3 to 5°C higher than they would
otherwise be. However, the present strength and structure of the MOC may not continue. The IPCC Fourth
Assessment Report (IPCC, 2007) concludes that there is greater than 90% chance that the AMOC will slow by up to
half by 2100, compared to pre-industrial levels, offsetting some of the warming over the European sector of the North
Atlantic, and contributing to the rate of Atlantic sea-level-rise. The IPCC also concluded that there is less than 10%
chance of abrupt changes during the 21st Century. Daily observations using the RAPID MOC mooring array at 26°N
are providing a continuous and growing time series of the AMOC strength and structure, the time series is not yet
sufficiently long to determine if there is a long-term trend in the AMOC. There was a significant reduction in the
AMOC in 2009-2010 that has since recovered. The relationships between the AMOC reduction and the anomalous
winter weather over the UK at the same time are not yet understood. Other observations do not at present provide
a coherent Atlantic wide picture of MOC variability, and there is little evidence of any long-term slowing. Despite
substantial progress over recent years in understanding and modelling the AMOC, projections of its future fate are
still subject to significant uncertainty.

/pdf/impacts-of-climate-change-on-the-atlantic-heat-conveyor-417yns9gip.pdf

Impacts of climate change on the Atlantic Heat Conveyor

CMIP6.CMIP.MOHC.HadGEM3-GC31-LL.1pctCO2

. The biogeochemical evaluation toolkit, BGC-val, is a model- and
grid-independent Python toolkit that has been built to evaluate marine
biogeochemical models using a simple interface. Here, we present the ideas
that motivated the development of the BGC-val software framework, introduce
the code structure, and show some applications of the toolkit using model
results from the Fifth Climate Model Intercomparison Project (CMIP5). A brief
outline of how to access and install the repository is presented in
Appendix  A , but the specific details on how to use the
toolkit are kept in the code repository. The key ideas that directed the toolkit design were model and grid
independence, front-loading analysis functions and regional masking,
interruptibility, and ease of use. We present each of these goals, why they
were important, and what we did to address them. We also present an outline of
the code structure of the toolkit illustrated with example plots produced by
the toolkit. After describing BGC-val, we use the toolkit to investigate the performance
of the marine physical and biogeochemical quantities of the CMIP5 models and
highlight some predictions about the future state of the marine ecosystem
under a business-as-usual CO2 concentration scenario (RCP8.5).

/pdf/bgc-val-a-model-and-grid-independent-python-toolkit-to-2zi4orinlj.pdf

BGC-val: a model- and grid-independent Python toolkit to evaluate marine biogeochemical models

http://www.met.reading.ac.uk/~till/lit/titz_grid_geometry.pdf

Till Kuhlbrodt

Papers

CMIP6.CMIP.MOHC.HadGEM3-GC31-LL.historical

Impacts of climate change on the Atlantic Heat Conveyor

CMIP6.CMIP.MOHC.HadGEM3-GC31-LL.1pctCO2

BGC-val: a model- and grid-independent Python toolkit to evaluate marine biogeochemical models

Grid geometry effects on convection in ocean climate models: a conceptual study