Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Climate-driven ecological thresholds in China’s drylands modulated by grazing

Abstract

Degradation of ecosystems can occur when certain ecological thresholds are passed below which ecosystem responses remain within ‘safe ecological limits’. Ecosystems such as drylands are sensitive to both aridification and grazing, but the combined effects of such factors on the emergence of ecological thresholds beyond which ecosystem degradation occurs has yet to be quantitatively evaluated. This limits our understanding on ‘safe operating spaces’ for grazing, the main land use in drylands worldwide. Here we assessed how 20 structural and functional ecosystem attributes respond to joint changes in aridity and grazing pressure across China´s drylands. Gradual increases in aridity resulted in abrupt decreases in productivity, soil fertility and plant richness. Rising grazing pressures lowered such aridity thresholds for most ecosystem variables, thus showing how ecological thresholds can be amplified by the joint effects of these two factors. We found that 44.4% of China’s drylands are unsuitable for grazing due to climate change-induced aridification, a percentage that may increase to 50.8% by 2100. Of current dryland grazing areas, 8.9% exceeded their maximum allowable grazing pressure. Our findings provide important insights into the relationship between aridity and optimal grazing pressure and identify safe operating spaces for grazing across China’s drylands.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Nonlinear responses of multiple ecosystem variables to the joint effects of aridity and grazing pressure.
Fig. 2: Combined effects of aridity and grazing on ecosystem structure and functioning across China’s drylands.
Fig. 3: Future changes and climate change vulnerability in China’s drylands.

Data availability

The datasets analysed in this study are publicly available, with data sources for each indicator described in the Data collection subsection of Methods in the paper and summarized in Supplementary Table 1. The data that support the findings of this study are available from figshare at https://doi.org/10.6084/m9.figshare.22678999.

Code availability

All data processing and analysis were conducted in ArcGIS (version 10.7), Microsoft Excel (version 2022), Origin (version 2022b), chngpt and gam packages in R (version 4.1.2) and MATLAB (version 2020a). The code used in this study is available from figshare at https://doi.org/10.6084/m9.figshare.22678999.

References

  1. Prăvălie, R. Drylands extent and environmental issues. A global approach. Earth Sci. Rev. 161, 259–278 (2016).

    Article  Google Scholar 

  2. Reynolds, J. F. et al. Global desertification: building a science for dryland development. Science 316, 847–851 (2007).

    Article  CAS  Google Scholar 

  3. Stringer, L. C. et al. Climate change impacts on water security in global drylands. One Earth 4, 851–864 (2021).

    Article  Google Scholar 

  4. Ahlström, A. et al. The dominant role of semi-arid ecosystems in the trend and variability of the land CO2 sink. Science 348, 895–899 (2015).

    Article  Google Scholar 

  5. Poulter, B. et al. Contribution of semi-arid ecosystems to interannual variability of the global carbon cycle. Nature 509, 600–603 (2014).

    Article  CAS  Google Scholar 

  6. Maestre, F. T. et al. Biogeography of global drylands. New Phytol. 231, 540–558 (2021).

    Article  Google Scholar 

  7. D’Odorico, P., Bhattachan, A., Davis, K. F., Ravi, S. & Runyan, C. W. Global desertification: drivers and feedbacks. Adv. Water Res. 51, 326–344 (2013).

    Article  Google Scholar 

  8. Huang, J., Yu, H., Dai, A., Wei, Y. & Kang, L. Drylands face potential threat under 2 °C global warming target. Nat. Clim. Change 7, 417–422 (2017).

    Article  Google Scholar 

  9. Huang, J., Yu, H., Guan, X., Wang, G. & Guo, R. Accelerated dryland expansion under climate change. Nat. Clim. Change 6, 166–171 (2016).

    Article  Google Scholar 

  10. Seddon, A. W., Macias-Fauria, M., Long, P. R., Benz, D. & Willis, K. J. Sensitivity of global terrestrial ecosystems to climate variability. Nature 531, 229–232 (2016).

    Article  CAS  Google Scholar 

  11. Berdugo, M. et al. Global ecosystem thresholds driven by aridity. Science 367, 787–790 (2020).

    Article  CAS  Google Scholar 

  12. Maestre, F. T. et al. Structure and functioning of dryland ecosystems in a changing world. Annu. Rev. Ecol. Evol. Syst. 47, 215–237 (2016).

    Article  Google Scholar 

  13. Berdugo, M., Kéfi, S., Soliveres, S. & Maestre, F. T. Plant spatial patterns identify alternative ecosystem multifunctionality states in global drylands. Nat. Ecol. Evol. 1, 0003 (2017).

    Article  Google Scholar 

  14. Wang, C. et al. Aridity threshold in controlling ecosystem nitrogen cycling in arid and semi-arid grasslands. Nat. Commun. 5, 4799 (2014).

    Article  CAS  Google Scholar 

  15. Delgado-Baquerizo, M. et al. Decoupling of soil nutrient cycles as a function of aridity in global drylands. Nature 502, 672–676 (2013).

    Article  CAS  Google Scholar 

  16. Scheffer, M., Carpenter, S., Foley, J. A., Folke, C. & Walker, B. Catastrophic shifts in ecosystems. Nature 413, 591–596 (2001).

    Article  CAS  Google Scholar 

  17. Scheffer, M. & Carpenter, S. R. Catastrophic regime shifts in ecosystems: linking theory to observation. Trends Ecol. Evol. 18, 648–656 (2003).

    Article  Google Scholar 

  18. Rockström, J. et al. A safe operating space for humanity. Nature 461, 472–475 (2009).

    Article  Google Scholar 

  19. Scheffer, M. et al. Early-warning signals for critical transitions. Nature 461, 53–59 (2009).

    Article  CAS  Google Scholar 

  20. Gaitán, J. J. et al. Aridity and overgrazing have convergent effects on ecosystem structure and functioning in Patagonian rangelands. Land Degrad. Dev. 29, 210–218 (2018).

    Article  Google Scholar 

  21. Lohmann, D., Tietjen, B., Blaum, N., Joubert, D. F. & Jeltsch, F. Shifting thresholds and changing degradation patterns: climate change effects on the simulated long‐term response of a semi‐arid savanna to grazing. J. Appl. Ecol. 49, 814–823 (2012).

    Article  Google Scholar 

  22. Ruppert, J. C. et al. Quantifying drylands’ drought resistance and recovery: the importance of drought intensity, dominant life history and grazing regime. Glob. Change Biol. 21, 1258–1270 (2015).

    Article  Google Scholar 

  23. Dudney, J. & Suding, K. N. The elusive search for tipping points. Nat. Ecol. Evol. 4, 1449–1450 (2020).

    Article  Google Scholar 

  24. Maestre, F. T. et al. Grazing and ecosystem service delivery in global drylands. Science 378, 915–920 (2022).

    Article  CAS  Google Scholar 

  25. Vandandorj, S., Eldridge, D. J., Travers, S. K. & Delgado‐Baquerizo, M. Contrasting effects of aridity and grazing intensity on multiple ecosystem functions and services in Australian woodlands. Land Degrad. Dev. 28, 2098–2108 (2017).

    Article  Google Scholar 

  26. Li, C. et al. Drivers and impacts of changes in China’s drylands. Nat. Rev. Earth Environ. 2, 858–873 (2021).

    Article  Google Scholar 

  27. Mirzabaev, A. et al. Cross-Chapter Paper 3: Deserts, Semi-Arid Areas and Desertification (Cambridge Univ. Press, 2022).

  28. IPCC: Summary for Policymakers. In Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems (eds Shukla, P. R. et al.) (2019).

  29. Prăvălie, R., Bandoc, G., Patriche, C. & Sternberg, T. Recent changes in global drylands: evidences from two major aridity databases. Catena 178, 209–231 (2019).

    Article  Google Scholar 

  30. Herrero‐Jáuregui, C. & Oesterheld, M. Effects of grazing intensity on plant richness and diversity: a meta‐analysis. Oikos 127, 757–766 (2018).

    Article  Google Scholar 

  31. Milchunas, D. G. & Lauenroth, W. K. Quantitative effects of grazing on vegetation and soils over a global range of environments. Ecol. Monogr. 63, 327–366 (1993).

    Article  Google Scholar 

  32. Milchunas, D. G., Sala, O. E. & Lauenroth, W. K. A generalized model of the effects of grazing by large herbivores on grassland community structure. Am. Nat. 132, 87–106 (1988).

    Article  Google Scholar 

  33. Oñatibia, G. R., Amengual, G., Boyero, L. & Aguiar, M. R. Aridity exacerbates grazing‐induced rangeland degradation: a population approach for dominant grasses. J. Appl. Ecol. 57, 1999–2009 (2020).

    Article  Google Scholar 

  34. Oñatibia, G. R., Boyero, L. & Aguiar, M. R. Regional productivity mediates the effects of grazing disturbance on plant cover and patch‐size distribution in arid and semi‐arid communities. Oikos 127, 1205–1215 (2018).

    Article  Google Scholar 

  35. Asner, G. P., Elmore, A. J., Olander, L. P., Martin, R. E. & Harris, A. T. Grazing systems, ecosystem responses, and global change. Annu. Rev. Environ. Res. 29, 261–299 (2004).

    Article  Google Scholar 

  36. Zhang, R., Wang, J. & Niu, S. Toward a sustainable grazing management based on biodiversity and ecosystem multifunctionality in drylands. Curr. Opin. Environ. Sustain. 48, 36–43 (2021).

    Article  Google Scholar 

  37. Cade, B. S. & Noon, B. R. A gentle introduction to quantile regression for ecologists. Front. Ecol. Environ. 1, 412–420 (2003).

    Article  Google Scholar 

  38. Tomal, J. H. & Ciborowski, J. J. Ecological models for estimating breakpoints and prediction intervals. Ecol. Evol. 10, 13500–13517 (2020).

    Article  Google Scholar 

  39. Hirota, M., Holmgren, M., Van Nes, E. H. & Scheffer, M. Global resilience of tropical forest and savanna to critical transitions. Science 334, 232–235 (2011).

    Article  CAS  Google Scholar 

  40. Zhang, J. et al. Water availability creates global thresholds in multidimensional soil biodiversity and functions. Nat. Ecol. Evol. 7, 1002–1011 (2023).

    Article  Google Scholar 

  41. Wu, B. et al. Essential dryland ecosystem variables. Curr. Opin. Environ. Sustain. 48, 68–76 (2021).

    Article  Google Scholar 

  42. Trabucco, A. & Zomer, R. J. Global Aridity Index and Potential Evapotranspiration (ET0) Climate Database v2. figshare https://figshare.com/articles/dataset/Global_Aridity_Index_and_Potential_Evapotranspiration_ET0_Climate_Database_v2/7504448/3 (2018).

  43. Liu, J. et al. Validation of Moderate Resolution Imaging Spectroradiometer (MODIS) albedo retrieval algorithm: dependence of albedo on solar zenith angle. J. Geophys. Res. Atmos. 114, D01106 (2009).

    Google Scholar 

  44. Abatzoglou, J. T., Dobrowski, S. Z., Parks, S. A. & Hegewisch, K. C. TerraClimate, a high-resolution global dataset of monthly climate and climatic water balance from 1958–2015. Sci. Data 5, 170191 (2018).

    Article  Google Scholar 

  45. Batjes, N. H. Harmonized soil property values for broad-scale modelling (WISE30sec) with estimates of global soil carbon stocks. Geoderma 269, 61–68 (2016).

    Article  CAS  Google Scholar 

  46. Tucker, C. J. & Sellers, P. Satellite remote sensing of primary production. Int. J. Remote Sens. 7, 1395–1416 (1986).

    Article  Google Scholar 

  47. Justice, C. et al. An overview of MODIS Land data processing and product status. Remote Sens. Environ. 83, 3–15 (2002).

    Article  Google Scholar 

  48. Masek, J. G. et al. A Landsat surface reflectance dataset for North America, 1990-2000. IEEE Geosci. Remote Sens. Lett. 3, 68–72 (2006).

    Article  Google Scholar 

  49. Chinese Academy of Sciences Data Center for Resources and Environmental Sciences (RESDC, accessed 1 October 2022); http://www.resdc.cn

  50. Rodriguez-Caballero, E. et al. Dryland photoautotrophic soil surface communities endangered by global change. Nat. Geosci. 11, 185–189 (2018).

    Article  CAS  Google Scholar 

  51. Ellis, E. C., Antill, E. C. & Kreft, H. All is not loss: plant biodiversity in the Anthropocene. PLoS ONE 7, e30535 (2012).

    Article  CAS  Google Scholar 

  52. Burrell, A. L., Evans, J. P. & Liu, Y. Detecting dryland degradation using time series segmentation and residual trend analysis (TSS-RESTREND). Remote Sens. Environ. 197, 43–57 (2017).

    Article  Google Scholar 

  53. Abel, C. et al. The human-environment nexus and vegetation-rainfall sensitivity in tropical drylands. Nat. Sustain. 4, 25–32 (2021).

    Article  Google Scholar 

  54. Xu, J., Chen, J., Liu, Y. & Fan, F. Identification of the geographical factors influencing the relationships between ecosystem services in the Belt and Road region from 2010 to 2030. J. Clean. Prod. 275, 124153 (2020).

    Article  Google Scholar 

  55. Gridded Livestock of the World 2007 (FAO, 2007).

  56. Robinson, T. P. et al. Mapping the global distribution of livestock. PLoS ONE 9, e96084 (2014).

    Article  Google Scholar 

  57. Naidoo, R. et al. Global mapping of ecosystem services and conservation priorities. Proc. Natl Acad. Sci. USA 105, 9495–9500 (2008).

    Article  CAS  Google Scholar 

  58. Ma, H. et al. The global distribution and environmental drivers of aboveground versus belowground plant biomass. Nat. Ecol. Evol. 5, 1110–1122 (2021).

    Article  Google Scholar 

  59. Fong, Y. et al. chngpt: Threshold regression model estimation and inference. BMC Bioinform. 18, 454 (2017).

    Article  Google Scholar 

  60. Muggeo, V. M. Segmented: an R package to fit regression models with broken-line relationships. R News 8, 20–25 (2008).

    Google Scholar 

  61. Hastie, T. J. in Statistical Models in S (eds Chamber, J. M. & Hastie, T. J.) 249–307 (Routledge, 2017).

  62. Digital Map Database of China. Provincial boundary. Harvard Dataverse https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/DBJ3BX (2020).

Download references

Acknowledgements

This research is jointly funded by the National Natural Science Foundation of China Project (grant 41991235), China’s Second Scientific Research Project on the Qinghai–Tibet Plateau (grant 2019QZKK0405-02) and the Fundamental Research Funds for the Central Universities. F.T.M. is supported by Generalitat Valenciana (CIDEGENT/2018/041), by the Spanish Ministry of Science and Innovation (EUR2022-134048) and by the contract between ETH Zurich and University of Alicante ‘Mapping terrestrial ecosystem structure at the global scale’. E.R.-C. was supported by the Ramon y Cajal fellowship (RYC2020-030762-I) and by the CRUST R-Forze (PID2021-127631NA-I00) project founded by FEDER/Ministerio de Ciencia e Inovacion-Agencia Estatal de Investigación. Many thanks to A. W. R. Seddon at University of Bergen (Norway) for sharing the global Vegetation Sensitivity Index dataset.

Author information

Authors and Affiliations

Authors

Contributions

C.L., B.F., S.W., F.T.M. and L.C.S. conceived and designed the study. C.L. carried out the calculations, drafted the figures and wrote the first draft of the paper. W.Z., Z.R., M.H. and Y.Z. undertook data analysis and figure reproduction. B.F., S.W., L.C.S., E.R.-C., B.W. and F.T.M. reviewed and edited the paper before submission. All authors made substantial contributions to the discussion of content.

Corresponding author

Correspondence to Bojie Fu.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Sustainability thanks Jabed Tomal and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Nonlinear responses of multiple ecosystem variables to aridity.

Examples of aridity thresholds observed for NDVI (a), vegetation cover (b), aboveground carbon density (c), root/shoot ratio (d), soil carbon content (e), soil nitrogen content (f), belowground carbon density (g), carbon sequestration (h), biocrust cover (i), interannual variation of precipitation (j), plant species richness (k), and vegetation sensitivity index (l). In (a.1) to (l.1), black dashed lines and black solid lines represent the smoothed trend fitted by a generalized additive model (GAM) and the linear fits at both sides of each threshold, respectively. The vertical grey dashed lines describe the aridity threshold identified. In (a.2) to (l.2), violin diagrams show bootstrapped slopes at the threshold of the two regressions existing at each side of the threshold values (red: regression before the threshold; yellow, after the threshold). Asterisks indicate significant differences when conducting a Mann-Whitney U test (two-sided) between before and after the threshold where: ***= P < 0.001.

Extended Data Fig. 2 Nonlinear responses of multiple ecosystem variables to grazing pressure.

Examples of grazing pressure thresholds observed for NDVI (a), vegetation sensitivity index (b), aboveground carbon density (c), plant species richness (d), belowground carbon density (e), and sensitivity of vegetation to precipitation (f). In (a.1) to (f.1), black dashed lines and black solid lines represent the smoothed trend fitted by a GAM and the linear fits at both sides of each threshold, respectively. The vertical grey dashed lines describe the aridity threshold identified. In (a.2) to (f.2), violin diagrams show bootstrapped slopes at the threshold of the two regressions existing at each side of the threshold values (red: regression before the threshold; yellow, after the threshold). Asterisks indicate significant differences when conducting a Mann-Whitney U test (two-sided) of the data below and above the threshold where: ***= P value < 0.001.

Extended Data Fig. 3 Predicted areas with the difference between maximum allowable grazing pressure and current grazing pressure.

a,b, For the contrasting effect (a) or synergistic effect (b) of aridity and grazing pressure in China’s drylands. The blue and brown shading denotes where the maximum allowable grazing pressure is higher and lower than the current grazing level, respectively. The red lines denote the baseline drylands in 1950–2000 that are not suitable for grazing and thus where grazing is not recommended, as their maximum allowable grazing pressure is equal to zero and the current grazing pressure leads thresholds to be crossed for ecosystem attributes. The grey shading denotes drylands where the land covers are cropland, wetland or urban areas. The unshaded areas are not drylands today and therefore are outside of the range. Credit: basemaps from the Global Aridity Index database42 and China Data Lab62.

Extended Data Fig. 4 Future changes and climate change vulnerability in China’s drylands.

a. Temporal variation in the mean aridity values across Chinese drylands. The thin solid lines and shading are mean values and the 95% confidence intervals of the 20 CMIP5 climate models, respectively. Bold solid lines show the aridity trends by twenty-year running means. b. Projections of changes in the mean areas of four dryland subtypes (units: percentage of China’s drylands) based on CMIP5 two representative concentration pathways (RCPs): RCP8.5 and RCP4.5 relative to the baseline period (1980–2014) for 2020–2060 and 2061–2100, respectively. Results are presented as mean values ± s.d. (n = 40). Error bars reflect the minimum and maximum number of area change with values equal to one standard derivation above and below the mean area change. The center of the error bar corresponds to the result calculated when the mean area change values are used. Changes include any transitions from the rest of three dryland subtypes to a subtype. Increased category includes any transitions to a subtype from the rest of three dryland subtypes (for example, from dry sub-humid, semi-arid and arid to hyper-arid). Decreased category includes any transitions from a subtype to the rest of three dryland subtypes (for example, from hyper-arid to dry sub-humid, semi-arid and arid).

Extended Data Fig. 5 Predicted areas with the difference between maximum allowable grazing pressure and current grazing pressure in China’s drylands under CMIP5 scenarios.

a-b: CMIP5 scenarios RCP4.5 (that is, assuming saturated increase in CO2 emissions); a and b are for 2020–2060 and 2061–2100 relative to the baseline period (1980–2014), respectively. c-d: CMIP5 scenarios RCP8.5 (that is, assuming sustained increase in CO2 emissions); c and d are for 2020–2060 and 2061–2100 relative to the baseline period (1980–2014), respectively. The blue and brown shading denotes where the maximum allowable grazing pressure is higher and lower than the current grazing level, respectively. The red lines denote the baseline drylands in 1950–2000 that are not suitable for grazing and thus where grazing is not recommended, as their maximum allowable grazing pressure is equal to zero and the current grazing pressure leads thresholds to be crossed for ecosystem attributes. The grey shading denotes drylands where the land covers are cropland, wetland or urban areas. The unshaded areas are not drylands today and therefore are outside of the range. Credit: basemaps from the Global Aridity Index database42 and China Data Lab62.

Extended Data Fig. 6 Predicted areas with the difference between maximum allowable grazing pressure and current grazing pressure when aridity and grazing acted synergistically.

a-b: CMIP5 scenarios RCP4.5 (that is, assuming saturated increase in CO2 emissions); a and b are for 2020–2060 and 2061–2100 relative to the baseline period (1980–2014), respectively. c-d: CMIP5 scenarios RCP8.5 (that is, assuming sustained increase in CO2 emissions); c and d are for 2020–2060 and 2061–2100 relative to the baseline period (1980–2014), respectively. The grey shading denotes the baseline drylands in 1950–2000 that are unsuitable for grazing. The blue and brown shading denotes where the maximum allowable grazing pressure is higher and lower than the current grazing level, respectively. The red lines denote the baseline drylands in 1950–2000 that are not suitable for grazing and thus where grazing is not recommended, as their maximum allowable grazing pressure is equal to zero and the current grazing pressure leads thresholds to be crossed for ecosystem attributes. The grey shading denotes drylands where the land covers are cropland, wetland or urban areas. The unshaded areas are not drylands today and therefore are outside of the range. Credit: basemaps from the Global Aridity Index database42 and China Data Lab62.

Extended Data Fig. 7 Predicted areas with the difference between maximum allowable grazing pressure and current grazing pressure when aridity and grazing acted in opposition.

a-b: CMIP5 scenarios RCP4.5 (that is, assuming saturated increase in CO2 emissions); a and b are for 2020–2060 and 2061–2100 relative to the baseline period (1980–2014), respectively. c-d: CMIP5 scenarios RCP8.5 (that is, assuming sustained increase in CO2 emissions); c and d are for 2020–2060 and 2061–2100 relative to the baseline period (1980–2014), respectively. The blue and brown shading denotes where the maximum allowable grazing pressure is higher and lower than the current grazing level, respectively. The red lines denote the baseline drylands in 1950–2000 that are not suitable for grazing and thus where grazing is not recommended, as their maximum allowable grazing pressure is equal to zero and the current grazing pressure leads thresholds to be crossed for ecosystem attributes. The grey shading denotes drylands where the land covers are cropland, wetland or urban areas. The unshaded areas are not drylands today and therefore are outside of the range. Credit: basemaps from the Global Aridity Index database42 and China Data Lab62.

Extended Data Table 1 The linear relationship between aridity (Ar) and maximum allowable grazing pressure (Gr) as determined by the two-dimensional threshold model which considers the combined effects of aridity and grazing pressure

Supplementary information

Supplementary Information

Supplementary Figs. 1–8, Tables 1–9 and Appendix 1.

Reporting Summary

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Li, C., Fu, B., Wang, S. et al. Climate-driven ecological thresholds in China’s drylands modulated by grazing. Nat Sustain 6, 1363–1372 (2023). https://doi.org/10.1038/s41893-023-01187-5

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41893-023-01187-5

This article is cited by

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing