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This is the final version. Available on open access from IOP Publishing via the DOI in this record ; To achieve the goals of the Paris Agreement requires deep and rapid reductions in anthropogenic CO2 emissions, but uncertainty surrounds the magnitude and depth of reductions. Earth system models provide a means to quantify the link from emissions to global climate change. Using the concept of TCRE - the transient climate response to cumulative carbon emissions - we can estimate the remaining carbon budget to achieve 1.5 or 2 °C. But the uncertainty is large, and this hinders the usefulness of the concept. Uncertainty in carbon budgets associated with a given global temperature rise is determined by the physical Earth system, and therefore Earth system modelling has a clear and high priority remit to address and reduce this uncertainty. Here we explore multi-model carbon cycle simulations across three generations of Earth system models to quantitatively assess the sources of uncertainty which propagate through to TCRE. Our analysis brings new insights which will allow us to determine how we can better direct our research priorities in order to reduce this uncertainty. We emphasise that uses of carbon budget estimates must bear in mind the uncertainty stemming from the biogeophysical Earth system, and we recommend specific areas where the carbon cycle research community needs to re-focus activity in order to try to reduce this uncertainty. We conclude that we should revise focus from the climate feedback on the carbon cycle to place more emphasis on CO2 as the main driver of carbon sinks and their long-term behaviour. Our proposed framework will enable multiple constraints on components of the carbon cycle to propagate to constraints on remaining carbon budgets. ; Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme ; European Union Horizon 2020

This is the final version. Available on open access from Frontiers Media via the DOI in this record ; Data Availability Statement: The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author. ; The two major Brazilian biomes, the Amazonia and the Cerrado (savanna), are increasingly exposed to fires. The Amazonian Forest is a fire sensitive ecosystem where fires are a typically rare disturbance while the Cerrado is naturally fire-dependent. Human activities, such as landscape fragmentation and land-use management, have modified the fire regime of the Cerrado and introduced fire into the Amazonian Forest. There is limited understanding of the role of landscape fragmentation on fire occurrence in the Amazonia and Cerrado biomes. Due to differences in vegetation structure, composition, and land use characteristics in each biome, we hypothesize that the emerging burned area (BA) patterns will result from biome-specific fire responses to fragmentation. The aim of this study was to test the general relationship between BA, landscape fragmentation, and agricultural land in the Amazonia and the Cerrado biomes. To estimate the trends and status of landscape fragmentation a Forest Area Density (FAD) index was calculated based on the MapBiomas land cover dataset for both biomes between 2002 and 2018. BA fraction was analyzed within native vegetation against the FAD and agricultural land fraction. Our results showed an increase in landscape fragmentation across 16% of Amazonia and 15% of Cerrado. We identified an opposite relationship between BA fraction, and landscape fragmentation and agricultural fraction contrasting the two biomes. For Amazonia, both landscape fragmentation and agricultural fraction increased BA fraction due to an increase of human ignition activities. For the Cerrado, on the other hand, an increase in landscape fragmentation and agricultural fraction caused a decrease in BA fraction within the native vegetation. For both biomes, we found that during drought years BA increases whilst the divergent trends driven by fragmentation in the two contrasting global biomes is maintained. This understanding will be critical to informing the representation of fire dynamics in fire-enable Dynamic Global Vegetation Models and Earth System Models for climate projection and future ecosystem service provision. ; Newton Fund ; ESA Climate Change Initiative ; European Union Horizon 2020 ; Natural Environment Research Council (NERC)

International audience ; Amidst declarations of planetary emergency and reports that the window for limiting climate change to 1.5 °C is rapidly closing, global average temperatures and fossil fuel emissions continue to rise. Global fossil CO2 emissions have grown three years consecutively: +1.5% in 2017, +2.1% in 2018, and our slower central projection of +0.6% in 2019 (range of –0.32% to 1.5%) to 37 ± 2 Gt CO2 (Friedlingstein et al 2019 Earth Syst. Sci. Data accepted), after a temporary growth hiatus from 2014 to 2016. Economic indicators and trends in global natural gas and oil use suggest a further rise in emissions in 2020 is likely. CO2 emissions are decreasing slowly in many industrialized regions, including the European Union (preliminary estimate of −1.7% [–3.4% to +0.1%] for 2019, −0.8%/yr for 2003–2018) and United States (−1.7% [–3.7% to +0.3%] in 2019, −0.8%/yr for 2003–2018), while emissions continue growing in India (+1.8% [+0.7% to 3.7%] in 2019, +5.1%/yr for 2003–2018), China (+2.6% [+0.7% to 4.4%] in 2019, +0.4%/yr for 2003–2018), and rest of the world ((+0.5% [−0.8% to 1.8%] in 2019, +1.4%/yr for 2003–2018). Two under-appreciated trends suggest continued long-term growth in both oil and natural gas use is likely. Because per capita oil consumption in the US and Europe remains 5- to 20-fold higher than in China and India, increasing vehicle ownership and air travel in Asia are poised to increase global CO2 emissions from oil over the next decade or more. Liquified natural gas exports from Australia and the United States are surging, lowering natural gas prices in Asia and increasing global access to this fossil resource. To counterbalance increasing emissions, we need accelerated energy efficiency improvements and reduced consumption, rapid deployment of electric vehicles, carbon capture and storage technologies, and a decarbonized electricity grid, with new renewable capacities replacing fossil fuels, not supplementing them. Stronger global commitments and carbon pricing would help implement ...

International audience ; Amidst declarations of planetary emergency and reports that the window for limiting climate change to 1.5 °C is rapidly closing, global average temperatures and fossil fuel emissions continue to rise. Global fossil CO2 emissions have grown three years consecutively: +1.5% in 2017, +2.1% in 2018, and our slower central projection of +0.6% in 2019 (range of –0.32% to 1.5%) to 37 ± 2 Gt CO2 (Friedlingstein et al 2019 Earth Syst. Sci. Data accepted), after a temporary growth hiatus from 2014 to 2016. Economic indicators and trends in global natural gas and oil use suggest a further rise in emissions in 2020 is likely. CO2 emissions are decreasing slowly in many industrialized regions, including the European Union (preliminary estimate of −1.7% [–3.4% to +0.1%] for 2019, −0.8%/yr for 2003–2018) and United States (−1.7% [–3.7% to +0.3%] in 2019, −0.8%/yr for 2003–2018), while emissions continue growing in India (+1.8% [+0.7% to 3.7%] in 2019, +5.1%/yr for 2003–2018), China (+2.6% [+0.7% to 4.4%] in 2019, +0.4%/yr for 2003–2018), and rest of the world ((+0.5% [−0.8% to 1.8%] in 2019, +1.4%/yr for 2003–2018). Two under-appreciated trends suggest continued long-term growth in both oil and natural gas use is likely. Because per capita oil consumption in the US and Europe remains 5- to 20-fold higher than in China and India, increasing vehicle ownership and air travel in Asia are poised to increase global CO2 emissions from oil over the next decade or more. Liquified natural gas exports from Australia and the United States are surging, lowering natural gas prices in Asia and increasing global access to this fossil resource. To counterbalance increasing emissions, we need accelerated energy efficiency improvements and reduced consumption, rapid deployment of electric vehicles, carbon capture and storage technologies, and a decarbonized electricity grid, with new renewable capacities replacing fossil fuels, not supplementing them. Stronger global commitments and carbon pricing would help implement such policies at scale and in time.

International audience ; Amidst declarations of planetary emergency and reports that the window for limiting climate change to 1.5 °C is rapidly closing, global average temperatures and fossil fuel emissions continue to rise. Global fossil CO2 emissions have grown three years consecutively: +1.5% in 2017, +2.1% in 2018, and our slower central projection of +0.6% in 2019 (range of –0.32% to 1.5%) to 37 ± 2 Gt CO2 (Friedlingstein et al 2019 Earth Syst. Sci. Data accepted), after a temporary growth hiatus from 2014 to 2016. Economic indicators and trends in global natural gas and oil use suggest a further rise in emissions in 2020 is likely. CO2 emissions are decreasing slowly in many industrialized regions, including the European Union (preliminary estimate of −1.7% [–3.4% to +0.1%] for 2019, −0.8%/yr for 2003–2018) and United States (−1.7% [–3.7% to +0.3%] in 2019, −0.8%/yr for 2003–2018), while emissions continue growing in India (+1.8% [+0.7% to 3.7%] in 2019, +5.1%/yr for 2003–2018), China (+2.6% [+0.7% to 4.4%] in 2019, +0.4%/yr for 2003–2018), and rest of the world ((+0.5% [−0.8% to 1.8%] in 2019, +1.4%/yr for 2003–2018). Two under-appreciated trends suggest continued long-term growth in both oil and natural gas use is likely. Because per capita oil consumption in the US and Europe remains 5- to 20-fold higher than in China and India, increasing vehicle ownership and air travel in Asia are poised to increase global CO2 emissions from oil over the next decade or more. Liquified natural gas exports from Australia and the United States are surging, lowering natural gas prices in Asia and increasing global access to this fossil resource. To counterbalance increasing emissions, we need accelerated energy efficiency improvements and reduced consumption, rapid deployment of electric vehicles, carbon capture and storage technologies, and a decarbonized electricity grid, with new renewable capacities replacing fossil fuels, not supplementing them. Stronger global commitments and carbon pricing would help implement such policies at scale and in time.

International audience ; Amidst declarations of planetary emergency and reports that the window for limiting climate change to 1.5 °C is rapidly closing, global average temperatures and fossil fuel emissions continue to rise. Global fossil CO2 emissions have grown three years consecutively: +1.5% in 2017, +2.1% in 2018, and our slower central projection of +0.6% in 2019 (range of –0.32% to 1.5%) to 37 ± 2 Gt CO2 (Friedlingstein et al 2019 Earth Syst. Sci. Data accepted), after a temporary growth hiatus from 2014 to 2016. Economic indicators and trends in global natural gas and oil use suggest a further rise in emissions in 2020 is likely. CO2 emissions are decreasing slowly in many industrialized regions, including the European Union (preliminary estimate of −1.7% [–3.4% to +0.1%] for 2019, −0.8%/yr for 2003–2018) and United States (−1.7% [–3.7% to +0.3%] in 2019, −0.8%/yr for 2003–2018), while emissions continue growing in India (+1.8% [+0.7% to 3.7%] in 2019, +5.1%/yr for 2003–2018), China (+2.6% [+0.7% to 4.4%] in 2019, +0.4%/yr for 2003–2018), and rest of the world ((+0.5% [−0.8% to 1.8%] in 2019, +1.4%/yr for 2003–2018). Two under-appreciated trends suggest continued long-term growth in both oil and natural gas use is likely. Because per capita oil consumption in the US and Europe remains 5- to 20-fold higher than in China and India, increasing vehicle ownership and air travel in Asia are poised to increase global CO2 emissions from oil over the next decade or more. Liquified natural gas exports from Australia and the United States are surging, lowering natural gas prices in Asia and increasing global access to this fossil resource. To counterbalance increasing emissions, we need accelerated energy efficiency improvements and reduced consumption, rapid deployment of electric vehicles, carbon capture and storage technologies, and a decarbonized electricity grid, with new renewable capacities replacing fossil fuels, not supplementing them. Stronger global commitments and carbon pricing would help implement such policies at scale and in time.

International audience ; Amidst declarations of planetary emergency and reports that the window for limiting climate change to 1.5 °C is rapidly closing, global average temperatures and fossil fuel emissions continue to rise. Global fossil CO2 emissions have grown three years consecutively: +1.5% in 2017, +2.1% in 2018, and our slower central projection of +0.6% in 2019 (range of –0.32% to 1.5%) to 37 ± 2 Gt CO2 (Friedlingstein et al 2019 Earth Syst. Sci. Data accepted), after a temporary growth hiatus from 2014 to 2016. Economic indicators and trends in global natural gas and oil use suggest a further rise in emissions in 2020 is likely. CO2 emissions are decreasing slowly in many industrialized regions, including the European Union (preliminary estimate of −1.7% [–3.4% to +0.1%] for 2019, −0.8%/yr for 2003–2018) and United States (−1.7% [–3.7% to +0.3%] in 2019, −0.8%/yr for 2003–2018), while emissions continue growing in India (+1.8% [+0.7% to 3.7%] in 2019, +5.1%/yr for 2003–2018), China (+2.6% [+0.7% to 4.4%] in 2019, +0.4%/yr for 2003–2018), and rest of the world ((+0.5% [−0.8% to 1.8%] in 2019, +1.4%/yr for 2003–2018). Two under-appreciated trends suggest continued long-term growth in both oil and natural gas use is likely. Because per capita oil consumption in the US and Europe remains 5- to 20-fold higher than in China and India, increasing vehicle ownership and air travel in Asia are poised to increase global CO2 emissions from oil over the next decade or more. Liquified natural gas exports from Australia and the United States are surging, lowering natural gas prices in Asia and increasing global access to this fossil resource. To counterbalance increasing emissions, we need accelerated energy efficiency improvements and reduced consumption, rapid deployment of electric vehicles, carbon capture and storage technologies, and a decarbonized electricity grid, with new renewable capacities replacing fossil fuels, not supplementing them. Stronger global commitments and carbon pricing would help implement such policies at scale and in time.

This is the final version. Available on open access via the DOI in this record ; Data availability: The datasets analysed during this study are available online: CMIP5 model output [https://esgf-node.llnl.gov/search/cmip5/], CMIP6 model output [https://esgf-node.llnl.gov/search/cmip6/], The WFDEI Meteorological Forcing Data [https://rda.ucar.edu/datasets/ds314.2/], CARDAMOM Heterotrophic Respiration [https://datashare.is.ed.ac.uk/handle/10283/875], MODIS Net Primary Production [https://lpdaac.usgs.gov/products/mod17a3v055/], Raich et al. 2002 Soil Respiration [https://cdiac.ess-dive.lbl.gov/epubs/ndp/ndp081/ndp081.html], Hashimoto et al. 2015 Heterotrophic Respiration [http://cse.ffpri.affrc.go.jp/shojih/data/index.html], and the datasets for observational Soil Carbon [https://github.com/rebeccamayvarney/soiltau_ec]. ; Code availability: The Python code used to complete the analysis and produce the figures in this study is available in the following online repository [https://github.com/rebeccamayvarney/soiltau_ec]. ; Carbon cycle feedbacks represent large uncertainties on climate change projections, and the response of soil carbon to climate change contributes the greatest uncertainty to this. Future changes in soil carbon depend on changes in litter and root inputs from plants, and especially on reductions in the turnover time of soil carbon (τs) with warming. The latter represents the change in soil carbon due to the response of soil turnover time (∆Cs,τ), and can be diagnosed from projections made with Earth System Models (ESMs). It is found to span a large range even at the Paris Agreement Target of 2◦C global warming. We use the spatial variability of τs inferred from observations to obtain a constraint on ∆Cs,τ . This spatial emergent constraint allows us to greatly reduce the uncertainty in ∆Cs,τ at 2◦C global warming. We do likewise for other levels of global warming to derive a best estimate for the effective sensitivity of τs to global warming, and derive a q10 equivalent value for heterotrophic respiration. ; European Research Council (ERC) ; Natural Environment Research Council (NERC) ; European Union Horizon 2020 ; Met Office Hadley Centre Climate Programme

This is the final version. Available on open access from IOP Publishing via the DOI in this record ; Data availability statement: The data that support the findings of this study are openly available at the following URL/DOI:10.5281/zenodo.5658676 ; Current estimates of the global land carbon sink contain substantial uncertainties on interannual timescales which contribute to a non-closure in the global carbon budget (GCB) in any given year. This budget imbalance (BIM) partly arises due to the use of imperfect models which are missing or misrepresenting processes. One such omission is the separate treatment of downward direct and diffuse solar radiation on photosynthesis. Here we evaluate and use an improved high-resolution (6-hourly), gridded dataset of surface solar diffuse and direct fluxes, over 1901-2017, constrained by satellite and ground-level observations, to drive two global land models. Results show that tropospheric aerosol-light interactions have the potential for substantial land carbon impacts (up to 0.4 PgCyr-1 enhanced sink) at decadal timescales, however large uncertainties remain, with models disagreeing on the direction of change in carbon uptake. On interannual timescales, results also show an enhancement of the land carbon sink (up to 0.9 PgCyr-1) and subsequent reduction in BIM by 55% in years following volcanic eruptions. We therefore suggest GCB assessments include this dataset in order to improve land carbon sink estimates. ; European Union Horizon 2020 ; Newton Fund

Published ; Journal Article ; © Author(s) 2015. There is a large amount of organic carbon stored in permafrost in the northern high latitudes, which may become vulnerable to microbial decomposition under future climate warming. In order to estimate this potential carbon-climate feedback it is necessary to correctly simulate the physical dynamics of permafrost within global Earth system models (ESMs) and to determine the rate at which it will thaw. Additional new processes within JULES, the land-surface scheme of the UK ESM (UKESM), include a representation of organic soils, moss and bedrock and a modification to the snow scheme; the sensitivity of permafrost to these new developments is investigated in this study. The impact of a higher vertical soil resolution and deeper soil column is also considered. Evaluation against a large group of sites shows the annual cycle of soil temperatures is approximately 25 % too large in the standard JULES version, but this error is corrected by the model improvements, in particular by deeper soil, organic soils, moss and the modified snow scheme. A comparison with active layer monitoring sites shows that the active layer is on average just over 1 m too deep in the standard model version, and this bias is reduced by 70 cm in the improved version. Increasing the soil vertical resolution allows the full range of active layer depths to be simulated; by contrast, with a poorly resolved soil at least 50 % of the permafrost area has a maximum thaw depth at the centre of the bottom soil layer. Thus all the model modifications are seen to improve the permafrost simulations. Historical permafrost area corresponds fairly well to observations in all simulations, covering an area between 14 and 19 million km2. Simulations under two future climate scenarios show a reduced sensitivity of permafrost degradation to temperature, with the near-surface permafrost loss per degree of warming reduced from 1.5 million km2 °C-1 in the standard version of JULES to between 1.1 and 1.2 million km2 °C-1 in the new model version. However, the near-surface permafrost area is still projected to approximately half by the end of the 21st century under the RCP8.5 scenario. ; The authors acknowledge financial support by the European Union Seventh Framework Programme (FP7/2007-2013) project PAGE21, under GA282700. E. Burke was supported by the joint UK DECC/Defra Met Office Hadley Centre Climate Programme GA01101. M. Langer is also supported by a post-doctoral research fellowship (A. v. Humboldt Foundation).

The Paris Agreement has opened debate on whether limiting warming to 1.5 °C is compatible with current emission pledges and warming of about 0.9 °C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 warming to less than 0.6 °C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers, increasing to 240 GtC with ambitious non-CO2 mitigation. We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0 °C above the mid-nineteenth century. If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5 °C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5 °C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.

The Paris Agreement has opened debate on whether limiting warming to 1.5°C is compatible with current emission pledges and warming of about 0.9°C from the mid-19th-century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 warming to less than 0.6°C in 66% of Earth System Model members of the CMIP5 ensemble with no mitigation of other climate drivers, increasing to 240GtC with ambitious non-CO2 mitigation. We combine a simple climatecarbon- cycle model with estimated ranges for key climate system properties from the IPCC 5th Assessment Report. Assuming emissions peak and decline to below current levels by 2030 and continue thereafter on a much steeper decline, historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), gives a likely range of peak warming of 1.2- 2.0°C above the mid-19th-century. If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5°C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250 GtC and unlikely greater than 540GtC. Hence limiting warming to 1.5°C is not yet a geophysical impossibility, but likely requires delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction-rates proving economically, technically or politically unfeasible.

The Paris Agreement has opened debate on whether limiting warming to 1.5 °C is compatible with current emission pledges and warming of about 0.9 °C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 warming to less than 0.6 °C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers, increasing to 240 GtC with ambitious non-CO2 mitigation. We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0 °C above the mid-nineteenth century. If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5 °C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5 °C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.

This is the final version of the article. Available from EGU via the DOI in this record. ; Current global models of the carbon (C) cycle consider only vertical gas exchanges between terrestrial or oceanic reservoirs and the atmosphere, thus not considering the lateral transport of carbon from the continents to the oceans. Therefore, those models implicitly consider all of the C which is not respired to the atmosphere to be stored on land and hence overestimate the land C sink capability. A model that represents the whole continuum from atmosphere to land and into the ocean would provide a better understanding of the Earth's C cycle and hence more reliable historical or future projections. A first and critical step in that direction is to include processes representing the production and export of dissolved organic carbon in soils. Here we present an original representation of dissolved organic C (DOC) processes in the Joint UK Land Environment Simulator (JULES-DOCM) that integrates a representation of DOC production in terrestrial ecosystems based on the incomplete decomposition of organic matter, DOC decomposition within the soil column, and DOC export to the river network via leaching. The model performance is evaluated in five specific sites for which observations of soil DOC concentration are available. Results show that the model is able to reproduce the DOC concentration and controlling processes, including leaching to the riverine system, which is fundamental for integrating terrestrial and aquatic ecosystems. Future work should include the fate of exported DOC in the river system as well as DIC and POC export from soil. ; The research leading to these results received funding from the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement no. 643052 (C-CASCADES project). We want to thank Altaf Arain, Tim Moore, and Gerd Glexiner for providing the DOC measurements. Ronny Lauerwald received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 703813 for the Marie Sklodowska-Curie European Individual Fellowship "C-Leak". Jing Tang is financed by a Marie Sklodowska-Curie Action Individual Fellowship (MABVOC: 707187) and supported by the Danish National Research Foundation (CENPERM DNRF100). Marta Camino-Serrano acknowledges funding from the European Research Council Synergy grant ERC-2013-SyG-610028 IMBALANCE-P.

Climate has a huge influence on the way we live. For example, it affects the crops we can grow and the diseases we might encounter in particular locations. It also determines the physical infrastructure we need to build to survive comfortably in the face of extremes of heat, cold, drought and flood. Human emissions of carbon dioxide and other greenhouse gases have changed the composition of the atmosphere over the last two centuries. This is expected to take Earth's climate out of the relatively stable range that has characterised the last few thousand years, during which human society has emerged. Measurements of ice cores and sea-floor sediments show that the current concentration of carbon dioxide, at just over 400 parts per million, has not been experienced for at least three million years. This causes more of the heat from the Sun to be retained on Earth, warming the atmosphere and ocean. The global average of atmospheric temperature has so far risen by about 1˚C compared to the late 19th century, with further increases expected dependent on the trajectory of carbon dioxide emissions in the next few decades. In 2013 and 2014 the Intergovernmental Panel on Climate Change (IPCC) published its fifth assessment report (AR5) assessing the evidence about climate change and its impacts. This assessment considered data from observations and records of the past. It then assessed future changes and impacts based on various scenarios for emissions of greenhouse gases and other anthropogenic factors. In 2015, almost every nation in the world agreed (in the so-called Paris Agreement) to the challenging goal of keeping global average warming to well below 2°C above pre-industrial temperatures while pursuing efforts to limit it to 1.5°C. With the next assessment report (AR6) not due until 2022, it is timely to consider how evidence presented since the publication of AR5 affects the assessments made then. The Earth's climate is a complex system. To understand it, and the impact that climate change will have, requires many different kinds of study. Climate science consists of theory, observation and modelling. Theory begins with well-established scientific principles, seeks to understand processes occurring over a range of spatial and temporal scales and provides the basis for models. Observation includes long time series of careful measurements, recent data from satellites, and studies of past climate using archives such as tree rings, ice cores and marine sediments. It also encompasses laboratory and field experiments designed to test and enhance understanding of processes. Computer models of the Earth climate system use theory, calibrated and validated by the observations, to calculate the result of future changes. There are nevertheless uncertainties in estimating future climate. Firstly the course of climate change is dependent on what socioeconomic, political and energy paths society takes. Secondly there remain inevitable uncertainties induced for example by variability in the interactions between different parts of the Earth system and by processes, such as cloud formation, that occur at too small a scale to incorporate precisely in global models. Assessments such as those of the IPCC describe the state of knowledge at a particular time, and also highlight areas where more research is needed. We are still exploring and improving our understanding of many of the processes within the climate system, but, on the whole, new research confirms the main ideas underpinning climate research, while refining knowledge, so as to reduce the uncertainty in the magnitude and extent of crucial impacts.