Policymakers are meeting in Paris for the latest round of international climate talks.

Ice sheets expanded into the tropics at least three times in Earth's history, covering the bulk — if not all — of the globe in ice. These Snowball Earth events reflect massive and unique perturbations to the Earth's climate-carbon system. In this focus, we bring together research and opinion pieces that explore the causes and consequences of Snowball Earth glaciations.

Seventeen goals to ensure the sustainable development of the planet have been identified by the world's political leaders, and they are ready to be adopted at a Summit in New York on 25 to 27 September. The challenges encompass environmental, economic and social aspects of one overarching aim: to allow humanity to thrive without depleting the Earth's resources. We present a collection of opinion pieces and primary research articles that illustrate the enormity and range of the tasks ahead.

Reforms in science education are aiming to teach scientists the skills they need for the twenty-first century, as documented in a NatureSpecial (nature.com/stem). It is important, too, to provide opportunities in the sciences for bright students from all backgrounds. In this web focus, we present a collection of opinion pieces that introduce a range of ideas for breaking down barriers to engaging in the geociences.

In April 1815, the eruption of Tambora VolcaNo. in Indonesia — one of the largest in recorded history — blasted ash and gases into the atmosphere purportedly causing widespread cooling and crop failure. 200 years on, the dynamics and effects of the Tambora eruption continue to fascinate and inform understanding of other giant eruptions in the past and future. In this Web Focus, we bring together a collection of opinion pieces that discuss current understanding of the Tambora event and other giant eruptions, and their impacts on society and the environment.

The demand for metals continues to grow, driven by the development of new technologies and the need for infrastructure to sustain ever-increasing populations. With improved understanding of the processes that transport and accumulate metals into economically viable deposits, we can target new places for exploration. In this Web Focus, we bring together a collection of primary research articles and opinion pieces that advance our understanding of how and where metals become enriched in Earth's crust and discuss strategies for their extraction.

In 1934, Alfred Redfield discovered that the ratio of carbon to nitrogen to phosphorus is a nearly constant 106:16:1 throughout the world's oceans, in both phytoplankton biomass and in dissolved nutrient pools. This insight has proved invaluable in understanding marine biogeochemical cycles, but, 80 years later, subtle variations to this ratio have emerged. In this Web Focus, we present a collection of research and opinion pieces that examine nutrient dynamics across ancient and modern changing environments.

Transparency and reproducibility are key ingredients of good science, and require that data and methods, including computer code, be made available. In this collection of opinion pieces, we highlight some of the chances and challenges in opening code and data to the scrutiny of the scientific community and the world at large.

The countdown is on to reach a legally binding agreement between all nations on actions to mitigate climate change. In 2011, the United Nations Climate Change Conference agreed that such a deal will be in place by 2015, and implemented by 2020. In this joint web Focus, timed to coincide with the New York Climate Summit, Nature GeoscienceandNature Climate Changepresent a series of overview articles and opinion pieces that take stock of emissions and climate change uncertainties and discuss potential ways forward.

The countdown is on to reach a legally binding agreement between all nations on actions to mitigate climate change. In 2011, the United Nations Climate Change Conference agreed that such a deal will be in place by 2015, and implemented by 2020. In this joint web Focus, timed to coincide with the New York Climate Summit, Nature GeoscienceandNature Climate Changepresent a series of overview articles and opinion pieces that take stock of emissions and climate change uncertainties and discuss potential ways forward.

The long-held notion of a bone-dry Moon was challenged in 2008, with the detection of water in some of the Apollo samples. Since then, lunar scientists have sought to understand how much water is in the Moon, where it is, and where it comes from. In this web focus, we present an overview article, research papers and opinion pieces that evaluate the evidence for water in the lunar interior and on the lunar surface and discuss its origin — whether it was added by cometary impacts, implanted by the solar wind, or indigenous to a Moon that may not, in fact, have formed dry.

If food production is to keep pace with the demands of an ever-expanding global population, agricultural systems must be modified to cope with the rising temperatures and increased prevalence of droughts, floods and other extreme weather events that are projected to ensue. A series of opinion pieces in this issue explore some of the ways in which productivity can be improved and food security safeguarded, be it through the direct involvement of those that work on the land, partnerships between remote investors and local land owners, or treatments tailored to the agricultural system in hand.