Citizens ask the expert in climate physics

24 11 2020

In the first of two consecutive interviews with climate-change experts, authors, editors and readers of the Spanish magazine Quercus have a chat with Ken Caldeira, a global-ecology researcher at the Carnegie Institution for Science (Washington, USA). His responses attest that the climate system is complex, and that we need to be practical in dealing with the planet’s ongoing climate emergency.

PhD in atmospheric sciences and professor at Stanford University (USA), Ken Caldeira has pioneered the study of ocean acidification and its impact on coral reefs (1) and geoengineering solutions to mitigate anthropogenic climate change by extracting carbon from the atmosphere and reflecting solar radiation (2, 3). He has also been part of the Intergovernmental Panel on Climate change (IPCC) and assessed zero-emissions scenarios (4, 5). To the right, Ken manoeuvers a drone while collecting aerial data from the Great Barrier Reef in Australia (6). Source.

SARS-Covid-19 is impacting the world. In our home country, Spain, scientists argue that (i) previous budget cuts in public health have weakened our capacity to tackle the pandemic (7), and (ii) the expert panels providing advice to our government should be independent of political agendas in their membership and decisions (8). Nevertheless, the Spanish national and regional governments’ data lack the periodicity, coherence, and detail to harness an effective medical response (9). Sometimes it feels as if politics partly operate by neglecting the science needed to tackle challenges such as the covid pandemic or climate change.

Having said that, even if a country has cultivated and invested in the best science possible, people have difficulties coming to terms with the idea that scientists work with probabilities of alternative scenarios. As much as there are different ways of managing a pandemic, scientists differ about how to mitigate the ecological, economic, and health impacts of a high-carbon society.

Thus, a more and more common approach is to make collective assessments (elicitations) by weighing different points of view across experts — for instance, to establish links between climate change and armed conflict (10) or to evaluate the role of nuclear energy as we transition to a low-carbon energy-production model (11). The overarching goal is to quantify consensus based on different (evidence-based) opinions.

The questions we here ask Ken Caldeira could well have different answers if asked of other experts. Still, as Ken points out, it is urgent that (of the many options available) we use the immense and certainty-proof knowledge we have already about climate change to take actions that work.

Interview done 23 January 2020 

We italicise each question and the name of the person asking the question and cite one to three relevant publications per question. For expanding on Ken Caldeira’s views on climate change, see a sample of his public talks here and here and newspaper articles here and here.

Read the rest of this entry »

Cartoon guide to biodiversity loss LXIII

26 10 2020

The sixth set of biodiversity cartoons for 2020. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

Read the rest of this entry »

Climate explained: humans have dealt with plenty of climate variability

23 09 2020
© Professor John Long, Flinders University, Author provided

(originally published on The Conversation)


How much climate variability have humans dealt with since we evolved and since we started settling (Neolithic times)? How important was migration to human survival during these periods?


The climate always fluctuates as variation in the Sun’s heat reaching Earth drives glacial-interglacial cycles. Over the past 420,000 years there have been at least four major transitions between ice ages and relatively warmer interglacial periods.

Modern humans emigrated from Africa to populate the rest of the globe between 120,000 and 80,000 years ago, which means our species has had to adapt to many massive climate transitions.


Warming and cooling

The Last Interglacial 129,000–116,000 years ago was a period of intense global warming (from around 2 ℃ higher than today to as much as 11 ℃ higher in the Arctic), leading to a large reduction of the Arctic, Greenland and Antarctic ice sheets, and a 6–9 m rise in sea level.

The front of a glacier breaking away and falling into the sea.
Arctic glaciers have melted before. Flickr/Kimberly Vardeman, CC BY

The Last Glacial Maximum from 26,500–19,000 years ago coincided with a large drop in atmospheric CO₂ and a 4.3 ℃ cooling globally.

Read more: Climate explained: will the tropics eventually become uninhabitable?


Low temperatures turned much of the world’s water into ice and expanded glaciers.

Read the rest of this entry »

Many animals won’t cope with climate change without access to ample drinking water

12 08 2020

Climate change implies change in temperature and water, and both factors shape species’ tolerances to thermal stress. In our latest article, we show that lack of drinking water maximises differences in tolerance to high temperatures among populations of Iberian lizard species.


Climate change is a multidimensional phenomenon comprising temporal and spatial shifts in both temperature and precipitation (1). How we perceive climate change depends on whether we measure it as shift in (i) mean conditions (e.g., the mean air temperature or rainfall over a decade within a given territory), (ii) magnitude or frequency of extreme conditions (e.g., the frequency of floods or tornados or the number of days with temperatures or rainfall above or below a given threshold), or (iii) speed at which mean or extreme conditions change in space and/or time.

In aquatic ecosystems, climate change further alters water acidity, oxygen dissolution and melting of ice. However, many people, including some scientists, tend to equate climate change erroneously with increased mean temperatures. Psychologists have made the semantic point that the use of the expressions climate change and global warming as synonyms can give mixed messages to politicians, and society in general, about how serious and complex the climate emergency we are facing really is (2, 3) — see NASA’s simple-worded account on the subject here.

In our latest article (4), we reviewed the ecological literature to determine to what extent ecologists investigating the tolerance of terrestrial animals to high temperatures have looked at thermal effects over water effects. It turns out, they were five times more likely to examine temperature over water.


Frequency of correlations between climate (air temperature versus precipitation) and tolerance to high temperature of terrestrial fauna in 64 papers published in the ecological literature (thickest link = 36, thinnest link = 2) following a systematic literature review in Scopus (4).

This is counterintuitive. Just imagine you have been walking under the sun for several hours on one of those dog days of summer, and you are offered to choose between a sunshade or a bottle of water. I’d bet you’d choose the bottle of water.

Read the rest of this entry »

Successful movers responding to climate change

16 06 2020

tropical fishes range shiftsEcologists often rely on measuring certain elements of a species’ characteristics, behaviour, or morphology to determine if these — what we call ‘traits’ — give them certain capacities to exploit their natural environments. While sometimes a bit arbitrarily defined, the traits that can be measured are many indeed, and sometimes they reveal rather interesting elements of a species’ resilience in the face of environmental change.

As we know, climate change is changing the way species are distributed around the planet, for the main (and highly simplified) reason that the environments in which they’ve evolved and to which they have adapted are changing.

In the simplest case, a warming climate means that there is a higher and higher chance you’ll experience temperatures that really don’t suit you that well (think of a koala or a flying fox baking in a tree when the thermometer reads +45° in the shade). Just like you seeking those nice, air-conditioned spaces on a scorcher of a day, species like to move to where conditions are more acceptable to their particular physiologies and behaviours.

When they can’t change fast enough, they go extinct.

Ecologists use life-history traits to predict which species have the highest probability of moving to new areas in response to climate change. Most studies into this phenomenon have largely ignored that range shifts in fact occur in sequential stages: (1) the species arrives in a new place for the first time, (2) its population increases in size (and extent), and (3) it can continue to persist in the new spot. Read the rest of this entry »

Cartoon guide to biodiversity loss LXI

31 05 2020

The fourth set of biodiversity cartoons for 2020. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

Read the rest of this entry »

Cartoon guide to biodiversity loss LX

8 04 2020

The third set of biodiversity cartoons for 2020 (plus a video treat). See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

Read the rest of this entry »

A plant’s adaptive traits don’t follow climate conditions as you might expect

27 03 2020


Just a quick post today, my last one for March. Like probably most of you, I’ve been trying to pretend to be as normal as possible despite the COVID-19 surrealism all around me. But even COVID-19 has shifted my research to a small degree.

But I’m not going to talk about the global pandemic right now (I can almost hear the collective sigh of relief). Instead, I’m going to go back to topic and discuss a paper that I’ve just co-authored.

Last year I went to China’s Yunnan Province where I met some fantastic colleagues at the Xishuangbanna Tropical Botanical Garden who were doing some very cool stuff with the variation in plant functional traits across environmental gradients.

Well, those colleagues invited me to participate in one those research projects, and I’m happy to say that the result has just been published in Forests.

Measuring the functional traits of different alpine trees species in the Changbai Mountains of far north-eastern China (no, I didn’t get to go there), the research set out to test how these varied among species and elevation.

Of course, one expects that different trees use different combinations of traits to survive the rigours of mountain life (high variation in temperature, freezing, wind, etc.), but generally speaking, you might expect things like xylem vessel diameter and density to change more or less monotonically (i.e., changing in a consistent manner as elevation rises or falls). This is because trees should adapt their traits to the local conditions as best they can. Read the rest of this entry »

Cartoon guide to biodiversity loss LIX

24 02 2020

The second set of six biodiversity cartoons for 2020. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

Read the rest of this entry »

Heat tolerance highly variable among populations and species

14 01 2020

Many ecological studies have examined the tolerance of terrestrial wildlife to high and low air temperatures over global scales (e.g., 1, 2, 3). This topic has been boosted in the last two decades by ongoing and predicted impacts of climate change on biodiversity (see summary of 2019 United Nation’s report here and here).

However, it is unfortunate that for most species, studies have measured thermal tolerance from a single location or population. Researchers interested in global patterns of thermal stress collect those measurements from the literature for hundreds to thousands of species [recently compiled in the GlobTherm database] (4), and are therefore often restricted to analysing one value of thermal tolerance per species.


Three of the 15 species of Iberian lacertids sampled in our study of thermal tolerance (9), including the populations of Algerian psammodromus (Psammodromus algirus), Geniez’s wall lizard (Podarcis virescens) and Western green lizard (Lacerta bilineata) sampled in Navacerrada (Madrid), Fuertescusa (Cuenca) and Moncayo (Soria), respectively. Photos by S. Herrando-Pérez

Using this approach, ecologists have concluded that cold tolerance is far more variable than heat tolerance across species from the tropics to the boreal zone (5-8). Consequently, tolerance to heat stress might be a species trait with limited potential to change in response to global warming compared to cold tolerance (5). Read the rest of this entry »

Cartoon guide to biodiversity loss LVIII

4 01 2020

The first set of six biodiversity cartoons for 2020. This special, Australia-is-burning-down-themed set is dedicated to Scott Morrison and his ilk. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

Read the rest of this entry »

Influential conservation ecology papers of 2019

24 12 2019

Bradshaw-Waves breaking on rocks Macquarie Island
As I’ve done for the last six years, I am publishing a retrospective list of the ‘top’ 20 influential papers of 2109 as assessed by experts in F1000 Prime (in no particular order). See previous years’ lists here: 20182017, 20162015, 2014, and 2013.







Read the rest of this entry »

Did people or climate kill off the megafauna? Actually, it was both

10 12 2019

When freshwater dried up, so did many megafauna species.
Centre of Excellence for Australian Biodiversity and Heritage, Author provided

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Earth is now firmly in the grips of its sixth “mass extinction event”, and it’s mainly our fault. But the modern era is definitely not the first time humans have been implicated in the extinction of a wide range of species.

In fact, starting about 60,000 years ago, many of the world’s largest animals disappeared forever. These “megafauna” were first lost in Sahul, the supercontinent formed by Australia and New Guinea during periods of low sea level.

The causes of these extinctions have been debated for decades. Possible culprits include climate change, hunting or habitat modification by the ancestors of Aboriginal people, or a combination of the two.

Read more: What is a ‘mass extinction’ and are we in one now?

The main way to investigate this question is to build timelines of major events: when species went extinct, when people arrived, and when the climate changed. This approach relies on using dated fossils from extinct species to estimate when they went extinct, and archaeological evidence to determine when people arrived.

Read more: An incredible journey: the first people to arrive in Australia came in large numbers, and on purpose

Comparing these timelines allows us to deduce the likely windows of coexistence between megafauna and people.

We can also compare this window of coexistence to long-term models of climate variation, to see whether the extinctions coincided with or shortly followed abrupt climate shifts.

Data drought

One problem with this approach is the scarcity of reliable data due to the extreme rarity of a dead animal being fossilised, and the low probability of archaeological evidence being preserved in Australia’s harsh conditions. Read the rest of this entry »

Climate change and humans together pushed Australia’s biggest beasts to extinction

25 11 2019

people-megafaunaOver the last 60,000 years, many of the world’s largest species disappeared forever. Some of the largest that we generally call ‘megafauna’ were first lost in Sahul — the super-continent formed by the connection of Australia and New Guinea during periods of low sea level. The causes of these extinctions have been heavily debated for decades within the scientific community.

Three potential drivers of these extinctions have been suggested. The first is climate change that assumes an increase in arid conditions that eventually became lethal to megafauna. The second proposed mechanism is that the early ancestors of Aboriginal people who either hunted megafauna species to extinction, or modified ecosystems to put the largest species at a disadvantage. The third and most nuanced proposed driver of extinction is the combination of the first two.

The primary scientific tools we scientists use to determine which of these proposed causes of extinction have the most support are dated fossil records from the extinct species themselves, as well as archaeological evidence from early Aboriginal people. Traditionally, the main way we use these data is to construct a timeline of when the last fossil of a species was preserved, and compare this to evidence indicating when people arrived. We can also reconstruct climate patterns back tens of thousands of years using models similar to the ones used today to predict future climates. Based on the comparison of all of these different timelines, we conclude that abrupt climate changes in the past were influential if they occurred at or immediately before a recorded extinction event. On the other hand, if megafauna extinctions occur immediately after humans are thought to have arrived, we attribute more weight to human arrival as a driver.

Read the rest of this entry »

What is a ‘mass extinction’ and are we in one now?

13 11 2019

(reproduced from The Conversation)

For more than 3.5 billion years, living organisms have thrived, multiplied and diversified to occupy every ecosystem on Earth. The flip side to this explosion of new species is that species extinctions have also always been part of the evolutionary life cycle.

But these two processes are not always in step. When the loss of species rapidly outpaces the formation of new species, this balance can be tipped enough to elicit what are known as “mass extinction” events.

Read more: Climate change is killing off Earth’s little creatures

A mass extinction is usually defined as a loss of about three quarters of all species in existence across the entire Earth over a “short” geological period of time. Given the vast amount of time since life first evolved on the planet, “short” is defined as anything less than 2.8 million years.

Since at least the Cambrian period that began around 540 million years ago when the diversity of life first exploded into a vast array of forms, only five extinction events have definitively met these mass-extinction criteria.

These so-called “Big Five” have become part of the scientific benchmark to determine whether human beings have today created the conditions for a sixth mass extinction.

An ammonite fossil found on the Jurassic Coast in Devon. The fossil record can help us estimate prehistoric extinction rates. Corey Bradshaw, Author provided

Read the rest of this entry »

Koala extinctions past, present, and future

12 06 2019


Photo by John Llewelyn

Koalas are one of the most recognised symbols of Australian wildlife. But the tree-living marsupial koala is not doing well throughout much of its range in eastern Australia. Ranging as far north as Cairns in Queensland, to as far west as Kangaroo Island in South Australia, the koala’s biggest threats today are undeniably deforestation, road kill, dog attacks, disease, and climate change.

With increasing drought, heatwaves, and fire intensity and frequency arising from the climate emergency, it is likely that koala populations and habitats will continue to decline throughout most of their current range.

But what was the distribution of koalas before humans arrived in Australia? Were they always a zoological feature of only the eastern regions?

The answer is a resounding ‘no’ — the fossil record reveal a much more complicated story.

Read the rest of this entry »

Action, not just science

25 02 2019

raised fistsIt has taken me a long time to decide to do this, but with role models like Claire Wordley, Alejandro Frid, and James Hansen out there, I couldn’t really find any more excuses.

Yes, I’ve been a strong advocate for action on biodiversity, environment and climate-change issues for a long time, and I’ve even had a few political wins in that regard with my writing and representation. I’ve even called out more than once for universities to embrace divestment from fossil fuels (to my knowledge, even my own university still has not).

While I still think these avenues are important, my ongoing observation is that things are getting worse politically, not better. That means that the normal armchair advocacy embraced by even the most outspoken academics is probably not going to be enough to elicit real political change that we — no, the planet — desperately needs.

Extinction-Rebellion-South-Australia2It is for this reason that I’ve joined the Extinction Rebellion (South Australia Chapter), especially after my friend and colleague, Dr Claire Wordley of the University of Cambridge, joined the UK Rebellion and wrote about her experiences on this very blog. That, coupled with my ongoing and mounting concern for the future Earth my daughter will inherit, requires me to take to the streets. Read the rest of this entry »

Thirsty forests

1 02 2019

Climate change is one ingredient of a cocktail of factors driving the ongoing destruction of pristine forests on Earth. We here highlight the main physiological challenges trees must face to deal with increasing drought and heat.

Forests experiencing embolism after a hot drought. The upper-left pic shows Scots (Pinus sylvestris) and black (P. nigra) pines in Montaña de Salvador (Espuñola, Barcelona, Spain) during a hot Autumn in 2015 favouring a massive infestation by pine processionary caterpillars (Thaumetopoea pityocampa) and tree mortality the following year (Lluís Brotons/CSIC in InForest-CREAF-CTFC). To the right, an individual holm oak (Quercus ilex) bearing necrotic branches in Plasencia (Extremadura, Spain) during extreme climates from 2016 to 2017, impacting more than a third of the local oak forests (Alicia Forner/CSIC). The lower-left pic shows widespread die-off of trembling aspen (Populus tremuloides) from ‘Aspen Parkland’ (Saskatchewan, Canada) in 2004 following extreme climates in western North America from 2001 to 2002 (Mike Michaelian/Canadian Forest Service). To the right, several dead aspens near Mancos (Colorado, USA) where the same events hit forests up to one-century old (William Anderegg).

A common scene when we return from a long trip overseas is to find our indoor plants wilting if no one has watered them in our absence. But … what does a thirsty plant experience internally?

Like animals, plants have their own circulatory system and a kind of plant blood known as sap. Unlike the phloem (peripheral tissue underneath the bark of trunks and branches, and made up of arteries layered by live cells that transport sap laden with the products of photosynthesis, along with hormones and minerals — see videos here and here), the xylem is a network of conduits flanked by dead cells that transport water from the roots to the leaves through the core of the trunk of a tree (see animation here). They are like the pipes of a building within which small pressure differences make water move from a collective reservoir to every neighbours’ kitchen tap.

Water relations in tree physiology have been subject to a wealth of research in the last half a decade due to the ongoing die-off of trees in all continents in response to episodes of drought associated with temperature extremes, which are gradually becoming more frequent and lasting longer at a planetary scale (1). 

Embolised trees

During a hot drought, trees must cope with a sequence of two major physiological challenges (2, 3, 4). More heat and less internal water increase sap tension within the xylem and force trees to close their stomata (5). Stomata are small holes scattered over the green parts of a plant through which gas and water exchanges take place. Closing stomata means that a tree is able to reduce water losses by transpiration by two to three orders of magnitude. However, this happens at the expense of halting photosynthesis, because the main photosynthetic substrate, carbon dioxide (CO2), uses the same path as water vapour to enter and leave the tissues of a tree.

If drought and heat persist, sap tension reaches a threshold leading to cavitation or formation of air bubbles (6). Those bubbles block the conduits of the xylem such that a severe cavitation will ultimately cause overall hydraulic failure. Under those conditions, the sap does not flow, many parts of the tree dry out gradually, structural tissues loose turgor and functionality, and their cells end up dying. Thus, the aerial photographs showing a leafy blanket of forest canopies profusely coloured with greys and yellows are in fact capturing a Dantesque situation: trees in photosynthetic arrest suffering from embolism (the plant counterpart of a blood clot leading to brain, heart or pulmonary infarction), which affects the peripheral parts of the trees in the first place (forest dieback).

Read the rest of this entry »

Influential conservation ecology papers of 2018

17 12 2018

For the last five years I’ve published a retrospective list of the ‘top’ 20 influential papers of the year as assessed by experts in F1000 Prime — so, I’m doing so again for 2018 (interesting side note: six of the twenty papers highlighted here for 2018 appear in Science magazine). See previous years’ posts here: 2017, 20162015, 2014, and 2013.

Read the rest of this entry »

With a Rebel Yell, Scientists Cry ‘No, no, more!’

29 11 2018

Adrenaline makes experiences hyper-real. Everything seems to move in slow motion, apart from my heart, which is so loud that I am sure people can hear it even over the traffic.

It’s 11:03 on a sunny November morning in central London. As the green man starts to shine, I walk into the middle of the road and sit down. On either side of me, people do the same. There can only be about 50 of us sitting on this pedestrian crossing, and I murmur ‘are we enough?’

‘Look behind you,’ says a new friend.

I turn. Blackfriar’s Bridge, usually covered in cars and buses, is filling with people. Citizens walking into the road and staying there, unfurling colourful flags with hourglass symbols on them. The police film us, standing close, but make no move to arrest anyone. Later, we discover that at least some of them encourage our disobedience.

Messages start coming in — 6,000 people are here, and we’ve blocked five bridges in central London with Extinction Rebellion, protesting for action to stop climate change and species extinctions. I’m a scientist participating in my first ever civil disobedience, and for me, this changes everything.


Left to right: protestors include kids, company directors, and extinct species.

What makes a Cambridge academic — and thousands of other people — decide that sitting in a road is their best chance of being heard? In short, nothing else has got us the emissions cuts we need. The declaration that global warming is real and that greenhouse-gas emissions need to be cut came in 1988, when I was a year old. Since then, scientists have continued to be honest brokers, monitoring greenhouse gases, running models, presenting the facts to governments and to the people. And emissions have continued to climb. The 2018 IPCC report that shocked many of us into action told us we have 12 years to almost halve emissions, or face conditions incompatible with civilisation. How did we end up here? Read the rest of this entry »