Back to the Future ‘Hothouse’

Our current global warming target and the trajectory it places us on, towards a future ‘Hothouse Earth’, has been the subject of much recent discussion, stimulated by a paper by Will Steffen and colleagues.  In many respects, the key contribution of this paper and similar work is to extend the temporal framing of our climate discussions, beyond 2100 for several centuries or more.  Analogously, it is useful to extend our perspective backwards to similar time periods, to reflect on the last time Earth experienced such a Hothouse state and what it means.

The Steffen et al paper allows for a variety of framings, all related to the range of natural physical, biological and chemical feedbacks that will amplify or mitigate the human intervention in climate.  [Note: the authors frame their paper around the concept of a limited number of steady state scenarios/temperatures for the Earth.  They then argue that aiming for 2C, potentially an unstable state, could trigger feedbacks tipping the world towards the 4C warmer Hothouse.  I find that to be somewhat simplistic given the diversity of climate states that have existed, if even transiently, over the past 15 million years, but that is a discussion for another day.] From my perspective, the most useful framing – and one that remains true to the spirit of the paper is this: We have set a global warming limit of 2C by 2100, with an associated carbon budget. What feedback processes will that carbon budget and warming actually unleash over the coming century,  how much additional warming will they add, and when?

That is a challenging set of questions that comes with a host of caveats, most related to the profound uncertainty in the interlinked biogeochemical processes that underpin climate feedbacks. For example, as global warming thaws the permafrost, will it release methane (with a high global warming potential than carbon dioxide)? Will the thawed organic matter oxidise to carbon dioxide or will it be washed and buried in the ocean? And will the increased growth of plants under warmer conditions lead instead to the sequestration of carbon dioxide? The authors refer to previous studies that suggest a permafrost feedback yielding an additional 0.1C warming by the end of the century; but there is great uncertainty in both the magnitude of that impact and its timing.

And timing is the great question at the heart of this perspective piece.  I welcome it, because too often our perspective is fixed on the arbitrary date of 2100, knowing full well that the Earth will continue to warm and ice continue to melt long after that date.  In this sense, Steffen et al is not a contradiction to what has been reported from the IPCC but an expansion on it.

Classically, we discuss these issues in terms of fast and slow feedbacks, but in fact there is a continuum between near instantaneous feedbacks and those that act over hundreds, thousands or even millions of years.  A warmer atmosphere will almost immediately hold more water vapour, providing a rapid positive feedback on warming – and one that is included in all of those IPCC projections.  More slowly, soil carbon, including permafrost, will begin to oxidise, with microbial activity stimulated and accelerated under warmer conditions – a feedback that is only just now being included in Earth system models.  And longer term, all manner of processes will come into play – and eventually, they will include the negative feedbacks that have helped regulate Earth’s climate for the past 4 billion years.

There is enough uncertainty in these processes to express caution in some of the press’s more exuberant reporting of this topic.  But lessons from the past certainly underscore the concerns articulated by Steffen et al.  We think that the last time Earth had 410 ppm CO2, a level similar to what you are breathing right now, was the Pliocene about 3 million years ago.  This was a world that was 1 to 2C warmer than today (i.e. 2 to 3C warmer than the pre-industrial Earth) and with sea levels about 10 m higher.  This suggests that we are already locked into a world that far exceeds the ambitions and targets of the Paris Agreement.  This is not certain as we live on a different planet and one where the great ice sheets of Greenland and Antarctica might not only be victims of climate change but climate stabilisers through ice-sheet hysteresis. And even if a Pliocene future is fixed, it might take centuries for that warming and sea level change to be realised.

But that analogue does suggest caution, as advocated by the Hothouse Earth authors.

It also prompts us to ask what the Earth was like the last time its atmosphere held about 500 ppm CO2, similar to the level needed to achieve the Paris Agreement to limit end-of-century warming below 2C.  A useful analogue for those greenhouse gas levels is the Middle Miocene Climate Optimum, which occurred from 17 to 14.7 million years ago.

Figure showing changes in ocean temperature (based on oxygen isotopic compositions of benthic foraminifera) and pCO2 over the past 60 million years (from Palaeo-CO2).  Solid symbols are from the d11B isotope proxy and muted symbols are from the alkenone-based algal carbon isotope fractional proxy. Note the spike in pCO2 associated with the MMCO at about 15 million years ago.

As one would expect for a world with markedly higher carbon dioxide levels, the Miocene was hotter than the climate of today.  And consistent with many of Steffen et al.’s arguments, it was about 4C hotter rather than a mere 2C, likely due to the range of carbon cycle and ice-albedo feedbacks they describe.  But such warmth was not uniform – globally warmer temperatures of 4C manifest as far hotter temperatures in some parts of the world and only slightly warmer temperatures elsewhere. Pollen and microbial molecular fossils from the North Sea, for example, indicate that Northern Europe experienced sub-tropical climates.

But what were the impacts of this warmth?  What is a 4C warmer world like?  To understand that, we also need to understand the other ways in which the Miocene world differed from ours, not just due to carbon dioxide concentrations but also the ongoing movement of the continents and the continuing evolution of life.  In both respects, the Miocene was broadly similar to today.  The continents were in similar positions, and the geography of the Miocene is one we would recognise. But there were subtle differences, including the ongoing uplift of the Himalayas and the yet-to-be-closed gateway between North and South America, and these subtle differences could have had major impacts on Asian climate and the North Atlantic circulation, respectively.

Similarly, the major animal groups had evolved by this point, and mammals had firmly established their dominance in a world separated by 50 million years from the dinosaurs.  Remnant groups from earlier times (hell pigs!) still terrorised the landscape, but many of the groups were the same or closely related to those we would recognise today.  And although hominins would not appear until the end of the Miocene, the apes had become well established, represented by as many as a 100 species. In the oceans, the differences were perhaps more apparent, the seas thriving with the greatest diversity of cetaceans in the history of our planet and associated with them the gigantic macro-predators such as Charcharadon megalodon (The MegTM).

Smithsonian mural showing Miocene Fauna and landscape.

But it is the plants that exhibit the most pronounced differences between modern and Miocene life. Grasses had only recent proliferated across the planet at the time of the MMCO, and the C4 plants had yet to expand to their current dominance. And in this regard, the long-term evolution of Earth’s climate likely played a crucial role.  There are about 8100 species of C4 plants (although this comprises only 3% of the plant species known to us) and most of these are grasses with other notable species being maize and sugar cane. They are distinguished from the dominant C3 plants, which comprise almost all other species, by virtue of their carbon dioxide assimilation biochemistry (the Hatch-Slack mechanism) and their leaf cellular physiology (the Kranz leaf anatomy).  It is a collective package that is exceptionally well adapted to low carbon dioxide conditions, and their global expansion about 7 million years ago was almost certainly related to the long-term decline in carbon dioxide from the high levels of the Middle Miocene. Although C4 plants only represent a small proportion of modern plant species, the Miocene world, bereft of them, would have looked far different than today – lacking nearly half of our modern grass species and by extension clear analogues to the vast African savannahs.

Aside from these, the most profound differences between the Miocene world and that of today would have been the direct impacts of higher global temperatures.  There is strong evidence that the Greenland ice sheet was far reduced in size compared to that of today, and its extent and even whether or not it was a persistent ice sheet or an ephemeral one remains the subject of debate. Similarly, West Antarctica was likely devoid of permanent ice, and the East Antarctic Ice Sheet was probably smaller – perhaps far smaller – than it is today.  And collectively, these smaller ice sheets were associated with a sea level that was about 40 m higher than that of today.

The hot Miocene world would have been different in other ways, including the hydrological cycle.  Although less studied than for other ancient intervals, it is almost certain that elevated warmth – and markedly smaller equator-to-pole temperature differences – would have impacted the global distribution of water.  More water was evidently exported to the high latitudes, resulting in a warmer and vegetated Antarctica where the ice had retreated. It was also likely associated with far more extreme rainfall events, with the hot air able to hold greater quantities of water.  More work is needed, but it is tempting to imagine the impact of these hot temperatures and extreme rainfall events.  They would have eroded the soil and flushed nutrients to the sea, perhaps bringing about the spread of anoxic dead zones, similar to the Oceanic Anoxic Events of the Mesozoic or the dead zones of modern oceans caused by agricultural run-off. Indeed, the Miocene is characterised by the deposition of some very organic-rich rocks, including the North Pacific Monterey Formation, speaking to the occurrence of reduced oxygen levels in parts of these ancient oceans.


It is unclear if our ambitions to limit global warming to 2C by the end of this century really have put us on a trajectory for 4C. It is unclear if we are destined to return to the Miocene.

But if so, the Miocene world is one both similar to but markedly distinct from our own – a world of hotter temperatures, extremes of climate, fewer grasslands, Antarctic vegetation, Arctic forests and far higher sea levels. Crucially, it is not the world for which our current society, its roads, cities, power plants, dams, borders, farmlands and treaties, has been designed.

Moreover, the MMCO Earth is a world that slowly evolved from an even warmer one over millions of years*; and that then evolved over further millions of years to the one in which we now inhabit. It is not a world that formed in a hundred or even a thousand years.  And that leaves us three final lessons from the past.  First, we do not know how the life of this planet, from coral reefs to the great savannahs, will respond to such geologically rapid change.  Second, we do not know how we will respond to such rapid change; if we must adapt, we must learn how to do so creatively, flexibly and equitably.  And third, it is probably not too late to prevent such a future from materialising, but even if it is, we still must act to slow down that rate of change to which we must adapt.

And we still must act to ensure that our future world is only 4C hotter and analogous to the Miocene; if we fail to act, the world will be even hotter, and we will have to extend our geological search 10s of millions of years further into the past, back to the Eocene, to find an even hotter and extreme analogue for our future Hothouse World.

*The final jump into the MMCO appears to have been somewhat more sudden, but still spanned around two-hundred thousand years.  A fast event geologically but not on the timescales of human history.

This blog is written by Cabot Institute member Professor Rich Pancost, Head of Earth Sciences at the University of Bristol. This blog has been reposted with kind permission from Rich’s original blog.

Rich Pancost

Belo Monte: there is nothing green or sustainable about these mega-dams


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Google Maps

There are few dams in the world that capture the imagination as much as Belo Monte, built on the “Big Bend” of the Xingu river in the Brazilian Amazon. Its construction has involved an army of 25,000 workers working round the clock since 2011 to excavate over 240m cubic metres of soil and rock, pour three million cubic metres of concrete, and divert 80% of the river’s flow through 24 turbines.


The dam is located about 200km before the 1,640km Xingu meets the Amazon. kmusserCC BY-SA

Costing R$30 billion (£5.8 billion), Belo Monte is important not only for the scale of its construction but also the scope of opposition to it. The project was first proposed in the 1970s, and ever since then, local indigenous communities, civil society and even global celebrities have engaged in numerous acts of direct and indirect action against it.

While previous incarnations had been cancelled, Belo Monte is now in the final stages of construction and already provides 11,233 megawatts of energy to 60m Brazilians across the country. When complete, it will be the largest hydroelectric power plant in the Amazon and the fourth largest in the world.

Indigenous protests against Belo Monte at the UN’s sustainable development conference in Rio, 2012. Fernando Bizerra Jr / EPA

A ‘sustainable’ project?

The dam is to be operated by the Norte Energia consortium (formed of a number of state electrical utilities) and is heavily funded by the Brazilian state development bank, BNDES. The project’s supporters, including the governments of the Partido dos Trabalhadores (Workers’ Party) that held office between 2003 and 2011, have justified its construction on environmental grounds. They describe Belo Monte as a “sustainable” project, linking it to wider policies of climate change mitigation and a transition away from fossil fuels. The assertions of the sustainability of hydropower are not only seen in Brazil but can be found across the globe – with large dams presented as part of wider sustainable development agendas.

With hydropower representing 16.4% of total global installed energy capacity, hydroelectric dams are a significant part of efforts to reduce carbon emissions. More than 2,000 such projects are currently funded via the Clean Development Mechanism of the 1997 Kyoto Protocol – second only to wind power by number of individual projects.

While this provides mega-dams with an environmental seal of approval, it overlooks their numerous impacts. As a result, dams funded by the CDM are contested across the globe, with popular opposition movements highlighting the impacts of these projects and challenging their asserted sustainability.

Beautiful hill, to beautiful monster

Those standing against Belo Monte have highlighted its social and environmental impacts. An influx of 100,000 construction and service workers has transformed the nearby city of Altamira, for instance.

Hundreds of workers – unable to find employment – took to sleeping on the streets. Drug traffickers also moved in and crime and violence soared in the city. The murder rate in Altamira increased by 147% during the years of Belo Monte construction, with it becoming the deadliest city on earth in 2015.

In 2013, police raided a building near the construction site to find 15 women, held against their will and forced into sex work. Researchers later found that the peak hours of visits to their building – and others – coincided with the payday of those working on Belo Monte. In light of this social trauma, opposition actors gave the project a new moniker: Belo Monstro, meaning “Beautiful Monster”.

The construction of Belo Monte is further linked to increasing patterns of deforestation in the region. In 2011, deforestation in Brazil was highest in the area around Belo Monte, with the dam not only deforesting the immediate area but stimulating further encroachment.

In building roads to carry both people and equipment, the project has opened up the wider area of rainforest to encroachment and illegal deforestation. Greenpeace has linked illegal deforestation in indigenous reserves – more than 200km away – to the construction of the project, with the wood later sold to those building the dam.

Brazil’s past success in reversing deforestation rates became a key part of the country’s environmental movement. Yet recently deforestation has increased once again, leading to widespread international criticism. With increasing awareness of the problem, the links between hydropower and the loss of the Amazon rainforest challenge the continued viability of Belo Monte and similar projects.

Big dams, big problems

While the Clean Development Mechanism focuses on the reduction of carbon emissions, it overlooks other greenhouse gases emitted by hydropower. Large dams effectively emit significant quantities of methane for instance, released by the decomposition of plants and trees below the reservoir’s surface. While methane does not stay in the atmosphere for as long as carbon dioxide (only persisting for up to 12 years), its warming potential is far higher.

Belo Monte has been linked to these methane emissions by numerous opposition actors. Further research has found that the vegetation rotting in the reservoirs of dams across the globe may emit a million tonnes of greenhouse gases per year. As a result, it is claimed that these projects are – in fact – making a net contribution to climate change.

Far from providing a sustainable, renewable energy solution in a climate-changed world, Belo Monte is instead cast as exacerbating the problem that it is meant to solve.

The ConversationBelo Monte is just one of many dams across the globe that have been justified – and funded – as sustainable pursuits. Yet, this conflates the ends with the means. Hydroelectricity may appear relatively “clean” but the process in which a mega-dam is built is far from it. The environmental credentials of these projects remain contested, with Belo Monte providing just one example of how the sustainability label may finally be slipping.

This blog is written by Cabot Institute member Ed Atkins, Senior Teaching Associate, School of Geographical Sciences, University of Bristol.  This article was originally published on The Conversation. Read the original article.

Ed Atkins

Will July’s heat become the new normal?

Saddleworth Moor fire near Stalybridge, England, 2018.  Image credit: NASA

For the past month, Europe has experienced a significant heatwave, with both high temperatures and low levels of rainfall, especially in the North. Over this period, we’ve seen a rise in heat-related deaths in major cities, wildfires in Greece, Spain and Portugal, and a distinct ‘browning’ of the European landscape visible from space.

As we sit sweltering in our offices, the question on everyone’s lips seems to be “are we going to keep experiencing heatwaves like this as the climate changes?” or, to put it another way, “Is this heat the new norm?”

Leo Hickman, Ed Hawkins, and others, have spurred a great deal of social media interest with posts highlighting how climate events that are currently considered ‘extreme’, will at some point be called ‘typical’ as the climate evolves.

As part of a two-year project on how future climate impacts different sectors (, my colleagues and I have been developing complex computer simulations to explore our current climate as well as possible future climates. Specifically, we’re comparing what the world will look like if we meet the targets set out in the Paris agreement: to limit the global average temperature rise to a maximum of 2.0 degrees warming above pre-industrial levels but with the ambition of limiting warming to 1.5 degrees.

The world is already around 1 degree warmer on average than pre-industrial levels, and the evidence to date shows that every 0.5 degree of additional warming will make a significant difference to the weather we experience in the future.

So, we’ve been able to take those simulations and ask the question: What’s the probability of us experiencing European temperatures like July 2018 again if:

  1. We don’t emit any further greenhouse gases and things stay as they are (1 degree above pre-industrial levels).
  2. Greenhouse gas emissions are aggressively reduced, restricting global average temperature rise to 1.5 degrees above pre-industrial levels.
  3. Greenhouse gas emissions are reduced to a lesser extent, restricting global average temperature rise by 2 degrees above pre-industrial levels.

What we’ve found is that European heat of at least the temperatures we have experienced this July are likely to re-occur about once every 5-6 years, on average, in our current climate. While this seems often, remember we have already experienced 1C of global increase in temperature. We’ve also considered the temperature over the whole of Europe, not just focusing on the more extreme parts of the heatwave. If we considered only the hottest regions, this would push our current temperature re-occurrence times closer to 10-20 years. However, using this Europe-wide definition of the current heat event, we find that in the 1.5C future world, temperatures at least this high would occur every other year, and in a 2C world, four out of five summers would likely have heat events that are at least as hot as our current one. Worryingly, our current greenhouse gas emission trajectory is leading us closer to 3C, so urgent and coordinated action is still needed from our politicians around the world.

Our climate models are not perfect, and they cannot capture all aspects of the current heatwave, especially concerning the large-scale weather pattern that ‘blocked’ the cooler air from ending our current heatwave. These deficiencies increase the uncertainty in our future projections, but we still trust the ball-park figures.

Whilst these results are not peer-reviewed, and should be considered as preliminary findings, it is clear that the current increased heat experienced over Europe has a significant impact on society, and that there will be even more significant impacts if we were to begin experiencing these conditions as much as our analysis suggests.

Cutting our emissions now will save us a hell of a headache later.

This blog is written by Dr Dann Mitchell (@ClimateDann) and Peter Uhe from the University of Bristol Geographical Sciences department and the Cabot Institute for the Environment.

Dann Mitchell

Cities’ contributions to the global SDGs: A Bristol view

Earlier this month, people from around the globe gathered in New York for the annual review of the world’s progress towards achieving the UN Sustainable Development Goals (SDGs), an event known as the ‘High Level Political Forum’ (HLPF). These globally-agreed goals were developed in 2015, providing a vision for what the world should look like in 2030. Covering all three dimensions of sustainability through 17 Goals, 169 targets and 244 indicators, the SDGs have been called ‘the closest thing the world has to a strategy’.

This year the HLPF focused on 6 of these Goals, including sustainable cities and communities, SDG 11. The inclusion of cities as a specific goal is a success, and it is the first time that a subnational unit has been included in a UN statistical reporting framework.

But cities have an important role to play in meeting all of the Goals, beyond just SDG11. Urbanisation is increasingly seen as a key cross cutting element in almost every aspect of sustainable development. Forecasts suggest that by 2050 almost 70% of the world’s people will live in cities. The concentration of people living and working in urban areas creates acute sustainable development challenges in cities. And what happens within individual cities can have far-reaching environmental impacts on resource use, pollution and carbon emissions in far-away places. Because local sustainable development challenges have national and even international implications, cities have the power and the opportunity to make progress towards the global SDGs, by tackling city-level challenges through innovative technical and organisational solutions.

Indeed, the 2017 HLPF declaration highlighted “the need to take appropriate action towards localizing and communicating the [SDGs] at all levels, from the national to the community and grassroots level […] Efforts should be made to reach out to all stakeholders, including subnational and local authorities.” (para 28)

So, to achieve the ambitious SDGs by 2030, cities must be fully engaged with all the goals, and can work with each other to share learnings, as well as interact at national and global policy levels. For example, New York City presented the first-ever official city-level review of progress towards the SDGs at the HLPF 2018 linked with their OneNYC approach – and invited other cities to work with them.

Despite Bristol’s many successes, we continue to face important challenges. Prominent among these is intense inequality across economic, social and environmental domains: such as income inequality, poor air quality and persistent gaps in health and education outcomes across the city. The SDGs offer a framework for taking on these challenges in an integrated way to achieve sustainable and inclusive prosperity that leaves no-one and nowhere – including nature – behind.

For the last few years, Bristol has been grappling with how it can best engage with the SDGs through an alliance of stakeholders from across the city. This work and their views have informed our ‘Driving the SDGs agenda at a city level in Bristol’ report, released during this year’s HLPF, where UK Stakeholders for Sustainable Development and partners launched an initial review of UK progress ‘Measuring Up’.

This tells the story of the Bristol SDG Alliance, formed in 2016 to advocate for the practical use of the SDGs in Bristol – to ‘localise’ the Goals to the city – and shares key learnings.

Hosted by Bristol Green Capital Partnership, in part because the SDG agenda integrates the environmental, social and economic dimensions of sustainability, the Alliance has submitted evidence to a parliamentary inquiry, commissioned an SDGs & Bristol report, and facilitated an innovative academic role to link SDG research and engagement in Bristol.

In this role, I have been able to work collaboratively with Bristol City Council on behalf of the Alliance to integrate the SDGs into the emerging One City Plan. In addition, many businesses and other organisations in the city appreciate the relevance of the SDGs to their work, such as Airbus and Triodos Bank, among others.

As we move forward, we will be grappling with some of the challenges facing other cities working to localise the SDGs. For example, how best to monitor progress.

This is a challenge even at the national level, with the UK’s national statistics office still working hard to assess and collect the data to report on the SDGs nearly 3 years after they were agreed – see the national reporting platform. Such monitoring challenges are more acute at a city level, with extra complexities and fewer resources available to address them.

For the SDGs to be achieved by 2030, challenges such as these will need to be overcome by cities. The theme for 2019’s SDG review is ‘inclusiveness and equality’, where the UK will also undertake its first official national review. Bristol is well-placed to contribute in 2019. Collectively the city may wish to follow New York’s initiative and report alongside the UK on our city’s progress next year.

This blog is written by Allan Macleod, SDG research and engagement associate working across Bristol Green Capital Partnership, Bristol City Council and the University of Bristol.  It has been reposted with kind permission from the Bristol Green Capital blog.  View the original blog.

Allan Macleod