Climate report: Apr, May 2020

A collection of global warming references from recent Science Magazine issues

A collection of Climate related articles from Science Magazine in the months of April and May 2020.

Articles from Science

⒈ Forests susceptible to rapid climate changes (Apr 2020)
⒉ Amid pandemic, India relaxes environmental assessment rules (May 2020)
⒊ Lethal levels of heat and humidity occurring sooner than expected (May 2020)
⒋ How much heat can the forests take? (May 2020)
⒌ Global warming is reducing tree cover (May 2020)

Articles from Science

⒈ Forests and drought (Apr 17, 2020)

A carcass of an elephant that succumbed to drought is seen under a tree in Hwange National Park, in Zimbabwe, on 12 November 2019. PHOTO: ZINYANGE AUNTONY/AFP VIA GETTY IMAGES

Article: “Hanging by a thread? Forests and drought” by Timothy J. Brodribb, Jennifer Powers, Hervé Cochard, and Brendan Choat https://science.sciencemag.org/content/368/6488/261.full.

Trees are the living foundations on which most terrestrial biodiversity is built.… The slow construction of these carbon-dense, woody skeletons leads to a slow generation time, leaving trees and forests highly susceptible to rapid changes in climate.… The emerging view [is] that, similar to corals, tree species have rather inflexible damage thresholds, particularly in terms of water stress…. This Review examines recent progress in our understanding of how the future looks for forests growing in a hotter and drier atmosphere.

Rising CO₂ is not good for trees because it dries out the soil and plans and “any increase in the rate of soil drying caused by elevated temperatures is likely to lead to increasing damage to standing forests during drought”.

Trees have thick tree trunks with wide canopies to capture sunlight. Trees have complicated structures that (called xylem cells) that draw water from the soil to the canopy. Dry conditions (“soil water deficit”) lead to blockage “ultimately killing the plant by completely severing the connection between soil and leaves.”

The below image (from the study) illustrates this for two plant species.

(A) A representation of the impact of drought on two tree species with different thresholds for drought-induced vascular damage. Different xylem cavitation thresholds determine the water potential (Ψ: water stress intensifies as water potential becomes more negative) causing tree mortality. Two lines indicate the oscillating water stress between day and night as the two species (indicated by small tree icons) dehydrate after the cessation of rainfall (data are from two trees from a dry forest site in Tasmania, Australia). The cavitation threshold and the rate of drying (dΨ/dt) both determine how many days into an acute drought each species will die. The taller species, which is more vulnerable to cavitation and faster drying, dies (indicated by an orange X) in week 2, whereas the shorter species survives until rainfall (indicated by the blue rectangle in week 3), enabling the tree to recover hydration. The proximity between the cavitation threshold and the lowest water potential during drought is known as the hydraulic safety margin. The dehydration rate is a product of a set of environmental and biological factors, many of which interact. Increasing temperature increases the rate of drying both directly and by interaction with biological factors, whereas CO₂ has the potential to reduce dehydration by its biological interaction with stomata and the photosynthetic rate. Image source: from the article.

Conclusion

[T]he extremely rapid pace of climate change appears to be introducing enormous instability into the mortality rates of global forests.… Most models predict major damage to forests in the next century if current climate trajectories are not ameliorated.… [M]ost observational data suggest that forest decline is well under way.

⒉ Amid pandemic, India relaxes environmental assessment rules (May. 7, 2020)

The Dibang Valley, a biodiversity hot spot in northeastern India, is threatened by a proposed hydropower dam. GOLDENTAKIN/FLICKR/CC 2.0

News Article: “India’s push to relax environmental assessment rules amid pandemic draws criticism” by Vaishnavi Chandrashekhar, May 7, 2020 https://www.sciencemag.org/news/2020/05/india-s-push-relax-environmental-assessment-rules-amid-pandemic-draws-criticism.

The Indian government relaxes several environmental assessment rules using the pandemic as excuse. India’s Ministry of Environment, Forest, and Climate Change has approved “a slew of mining, infrastructure, and industrial projects, many of them in forest areas”. This includes “a new coal mine in an elephant reserve, preliminary drilling inside a wildlife sanctuary that is home to endangered lion-tailed macaques and great Indian hornbills” and “is considering two other controversial projects: a large hydropower project in the Dibang Valley, a biodiversity hot spot in northeastern India; and a uranium mine in a tiger reserve in central India”.

Kohli speaks
Kanchi Kohli, an environmental governance expert with the Centre for Policy Research says, “Considered decision-making with diverse points of view has been wilting over the past few years. The focus is on approval rather than scrutiny or discussion”.

Analysts note that the government’s push to approve industrial projects even during the pandemic is consistent with Prime Minister Narendra Modi’s pro-business stance. For example, an analysis by the Legal Initiative for Forest and Environment shows that from January to June 2019, a key panel overseeing wildlife sanctuaries and parks approved 63 of 70 development proposals, resulting in reduced protections for 216 hectares of land. The panel has just one independent scientist.

The tribals who live in the forest, the Idu-Mishmi tribe, oppose the exploitation of their lands. But clearly, the government has other interests.

⒊ Lethal levels of heat and humidity occurring sooner than expected (May 8, 2020)

Amritsar, India, is one place that has reeled under higher than normal heat and humidity. MUNISH BYALA/HINDUSTAN TIMES VIA GETTY IMAGES

News article: “Lethal levels of heat and humidity are gripping global ‘hot spots’ sooner than expected”, by Warren Cornwall, May. 8, 2020 https://www.sciencemag.org/news/2020/05/lethal-levels-heat-and-humidity-are-gripping-global-hot-spots-sooner-expected.

Research article: “The emergence of heat and humidity too severe for human tolerance”, by Colin Raymond, Tom Matthews, and Radley M. Horton https://advances.sciencemag.org/content/6/19/eaaw1838.full

There were 2x more heat hotspots in the world in 2017 than in 1979. Scientists measured wet bulb temperatures (see below) across the world and found these occur more frequently than before. Worse still, the increase in frequency is earlier than expected.

What is a wet bulb temperature? A “wet bulb temperature [is] the lowest temperature to which air can be cooled via evaporation”.

At wet bulb temperatures above 35°C, researchers estimate that even fit people will overheat and potentially die within 6 hours. Although that temperature might seem low, it equates to almost 45°C at 50% humidity, and what it would feel like 71°C using the U.S. National Weather Service heat index. In the [2003] heat wave [in which 70,000+ people died] that ravaged Europe, wet bulb temperatures hit 28°C.

Interactive map by the researchers:

Interactive Map: Daily Maximum Wet-Bulb Temperature (°C)

Conclusion

Our survey of the climate record from station data reveals many global TW exceedances of 31° and 33°C and two stations that have already reported multiple daily maximum TW values above 35°C. These conditions, nearing or beyond prolonged human physiological tolerance, have mostly occurred only for 1- to 2-hours’ duration (fig. S2). They are concentrated in South Asia, the coastal Middle East, and coastal southwest North America

⒋ How much heat can the forests take? (May 22, 2020)

Throughout the tropics, carbon stocks in forests, such as this one in Liberia, will be reduced in response to higher daytime temperatures. CREDIT: FABIAN PLOCK / ALAMY STOCK PHOTO

News article: “Thermal sensitivity of tropical trees”, by Andrew M. Sugden, https://science.sciencemag.org/content/368/6493/840.1

How much heat can the forests take? Forests are going to be good carbon absorbing systems (“carbon stocks”) under moderate climate change “if they are protected from direct impacts such as clearance, logging, or fires”.

Their capacity is “dominated by high daytime temperatures”. When temperatures rise, the trees get dry which leads to the problems described in ⑴ above. The researchers say temperatures above 32°C risk “greater loss of tropical forest carbon stocks”.

Science Article: “Long-term thermal sensitivity of Earth’s tropical forests”, Martin J. P. Sullivan, et al. https://science.sciencemag.org/content/368/6493/869.

Forests do okay with temperature increases if the moisture level is acceptable. They also do okay if only night-time temperatures rise. (In scientist-speak: “The effect of temperature on carbon residence time only emerges when dry-season precipitation is low” and “our results suggest that tropical forests have considerable potential to acclimate and adapt to the effects of nighttime minimum temperatures but are clearly sensitive to the effects of daytime maximum temperature” respectively.)

Temperature rises of 2°C above pre-industrial levels will push 71% of the tropical forests to levels where they no longer retain CO₂. (Tropical forests store 40% of the world’s vegetation carbon.)

Conclusion?
We must limit global temperature rises and perform “large-scale conservation and restoration to protect biodiversity”.

⒌ Global warming is reducing tree cover (May 29, 2020)

Science article: “Pervasive shifts in forest dynamics in a changing world”, by Nate G. McDowell, et al., May 29, 2020 https://science.sciencemag.org/content/368/6494/eaaz9463.

The abstract summarizes the problem succinctly:

[T]he evidence reveals that it is highly likely that tree mortality rates will continue to increase, whereas recruitment and growth will respond to changing drivers in a spatially and temporally variable manner. The net impact will be a reduction in forest canopy cover and biomass. [my emphasis]

The below image, from the article, illustrates their thesis. Read the description under the image.

In the far-left panel, a mature ecosystem is responsive primarily to localized mortality, and the primary drivers of demography are chronically changing variables such as CO₂, temperature, and vapor pressure deficit (VPD). In the next panel, the system is disturbed by fire, insect outbreak, or another large-scale perturbation that removes most of the overstory trees, and species adapted to rapid postdisturbance recruitment become established. In the third panel, recruitment and growth dominate demographic processes, with mortality increasing over time as competition leads to self-thinning. In the last panel, a mature ecosystem is dominated by species that have replaced the original community in response to chronic environmental changes, leading to a novel ecosystem.

When a mature forest suffers from various causes (like drought) the recovered forest has a smaller canopy. A smaller canopy means the forest both provides lesser of shade, heat absorption, and oxygen production, but it also is a smaller carbon sink.

Why are the resulting trees of a shorter canopy? In drier conditions (see ⑴ and ⑷ in this post) there trees have a harder time taking water from the roots to the canopy. (This article refers to this as a “vapor pressure deficit (VPD)”.) They also say “rising temperature forces an exponential rise in VPD, which prompts stomatal closure and limits photosynthesis, leading to lower growth, higher mortality (58), and reduced regeneration (59) and ultimately driving community shifts. These observations are consistent with hydraulic theory, which suggests that as VPD rises, potential maximum tree height declines”.

Wildfire
In Sep 2017, I wrote, “Forest Fires are getting worse” and concurring with me (ha, ha), the authors confirm that wildfires are getting worse worldwide. More seriously, they reference a Jul 2015 study by Jolly et al. (ref: https://www.nature.com/articles/ncomms8537), whose abstract says, quite alarmingly:

Climate strongly influences global wildfire activity…. We show that fire weather seasons have lengthened across 29.6 million km² (25.3%) of the Earth’s vegetated surface, resulting in an 18.7% increase in global mean fire weather season length. We also show a doubling (108.1% increase) of global burnable area affected by long fire weather seasons and an increased global frequency of long fire weather seasons across 62.4 million km² (53.4%) during the second half of the study period. If these fire weather changes are coupled with ignition sources and available fuel, they could markedly impact global ecosystems, societies, economies and climate.