Global Warming: Causes And Consequences

by Lucy Shapiro, Harley McAdams
Monday, April 8, 2019
Image credit: 
istock

Spaceship Earth

The familiar photo of the Earth spinning in the blackness of space that was taken 50 years ago by William Anders, an astronaut on the Apollo 8 lunar mission, starkly illustrated our isolation on this planet. Now we face a crisis as the climate and environmental conditions that support life as we know it become ever more fragile owing to CO2-induced global warming. The evidence suggests there is significant risk that areas of the Earth in tropical zones may become uninhabitable and that significant food chains will collapse in this century. We agree with those who say that the highest human priority now is to greatly reduce human societies’ reliance on CO2-producing oil and coal. However, even the most optimistic projections of reduced CO2 production and resulting reductions in climatic warming suggest that future generations will face daunting problems. Fortunately, this growing disruption is occurring at a time of unprecedented breakthroughs in science and technology. Although there are many things that can be done to ameliorate individual events, the worldwide effort is uncoordinated and there is widespread resistance from vested economic and political interest groups. Here, we first survey the consequences of the rapid rise in CO2 emissions and then consider the possibility that new genetic technologies can help mitigate some of the biological consequences of global changes in climate patterns.

Life on Earth has evolved in an interconnected ecology determined by weather patterns, movements of global tectonic plates, and the dynamic surface chemistry of oceans and land. The creatures on Earth—all the humans, animals, plants, bacteria, fungi, and viruses—are dependent on each another as well as on this enveloping ecosystem. Since the Earth is an integrated system, significant changes in any internal component or in external influences induce movement toward a new equilibrium. Throughout the history of the Earth there have been long periods of cooling leading to growth of massive continental ice sheets, interspersed with warm intervals. While the causes of these ice ages are not fully understood, the principal contributing factors have been identified. The composition of the atmosphere, particularly the concentration of carbon dioxide and methane, is important. Also changes in the Earth’s orbit around the sun, changes in the tilt in the Earth’s axis, impacts of large meteorites, and eruptions of super volcanoes. The latter two phenomena can both put massive amounts of particulate matter and carbon dioxide into the atmosphere.

In two instances, biological phenomena have disrupted the composition of the atmosphere with global consequences. One was the Great Oxidation Event or the Oxidation Catastrophe, around 2.45 billion years ago. This occurred after a bacterial species, an ancestor of contemporary cyanobacteria, evolved the ability to produce oxygen as a byproduct of photosynthesis. This event had extraordinary consequences for ocean chemistry and eventually for the slow accumulation of atmospheric oxygen to contemporary levels over an interval of several million years. The newly oxygenated atmosphere was toxic to virtually all the anaerobic organisms that then populated the earth. These organisms died and were replaced by creatures that could thrive in the new oxygenated atmosphere.1 Now, the current human-induced increase in atmospheric CO2 is the second biological disruption of atmospheric composition that is producing global warming with credible predictions of ever more dire consequences in coming decades. Consequences we are already seeing include:

Accelerating rise in global sea level owing to irreversible melting of glacial ice in the European Alps, melting of arctic ice, and of greatest concern, melting of the land ice sheets in Greenland and Antarctica.

Large changes in climate patterns that have led to cataclysmic wild fires encouraged by the hottest summers on record and extreme floods stemming from new and disruptive storm patterns.

Acidification and warming of the oceans leading to decimation of coral reefs and other changes that are disrupting the marine food chain.

The global redistribution of bacterial, fungal, and viral pathogens and their vectors out of the tropics and into temperate zones and the emergence of previously unknown pathogens.

As the Earth’s climate continues to warm owing to increasing levels of atmospheric CO2 the mean sea level will rise.2 The mean sea level has risen about 8 inches since the late 1800s, and projections suggest an accelerating rise of between 2 and 6 feet by 2100.3 The predominant contributor to the future sea level increase will be melting of the enormous land-based ice sheets and glaciers on Antarctica and Greenland. The amount of the rise will be strongly dependent on mankind’s success in limiting future CO2 emissions. However, even the lowest estimates portend devastating consequences:4 loss of arable land owing to flooding and salt water intrusion (e.g., Vietnam, Bangladesh, California’s Salinas valley5); major population displacements (100 million people will be displaced by a three-foot rise); many coastal areas may have to be abandoned (e.g., South Florida and Miami6).

We are already experiencing changes in global weather patterns. Regions accustomed to temperate temperatures and predictable periods of rainfall are seeing prolonged drought and periods of extreme high temperature, while other regions are experiencing excess rain and snowfall along with lower ambient temperatures. In parts of Australia, drought and peak summer temperatures nearing 116oF are causing vast wildfires. Simultaneously, U.S. states around the Great Lakes have experienced winter temperatures of -34oC (-29.2oF) that are significantly colder than temperatures in the Arctic. This skewing of ambient temperatures in North America is due to changes in the jet stream that have allowed polar air from the Arctic to flow into zones normally buffered against temperature extremes. Global warming contributes to these unusual weather patterns through its influence on the polar vortex, a wide expanse of swirling cold air near the pole.7 Over a surprisingly short time, the average temperature rise at the north polar region has been higher than in some more southerly areas. While average temperatures across the globe have now increased to 1.2oC above preindustrial revolution levels, the poles have seen an average increase of 3oC. During March 2018, temperatures in Siberia were 15oC (59oF) above historical averages, and Greenland experienced a period of 61 hours above freezing (three times longer than any previous year), while temperatures were unusually low in Europe. These disruptions in global weather patterns have caused long-term drought conditions in some regions and unprecedented floods in others, leading to loss of arable land and precipitous reductions in agricultural production. Those who deny climate change often point to periods of extreme cold in unexpected regions as evidence supporting their views, without understanding that the large-scale changes in weather patterns are a central consequence of global warming. When the oceans warm, global weather patterns are disrupted in many areas in unexpected ways.

It is important to recognize that these global events are interconnected. For example, consider the consequences of sustained rainfall on degraded farmland: Increased rainfall leads to soil erosion, that in turn results in the release of phosphorous from fertilized soil into rivers and the oceans. That release, in turn can stimulate algal blooms and red tides, further reducing the ocean oxygen levels that are already lowered by warming waters. These phenomena add to the impacts of warming and acidification on food chains in the ocean.

What will be the impact of global warming on our land-based food supply and our ability to maintain the animals and plants we depend on? Warming is already slowing yield gains in most wheat-growing locations, and global wheat production is expected to fall by 6% for each 1°C of further temperature increase while becoming more variable.8 Global production of corn is similarly at risk.9 Global warming will alter world food production patterns, with crop productivity reduced in low latitudes and tropical regions but increased somewhat in high latitude regions. This will lead to trade changes with expanded sales of food products from the mid-to-high latitudes to lower latitude regions.10

Extinction of species owing to expanding human activities around the globe has been accelerating over the last two centuries. Now the onset of changes in the climate is accelerating the rate of extinctions. Disruptions of habitats, loss of food sources, and the spread of infectious diseases are happening at a rate that cannot be accommodated by evolutionary adaptation. The number of species that have gone extinct in the last century alone would have taken between 800 and 1000 years to disappear in previous mass extinctions.11 During one of these extinctions, the Permian-Triassic extinction 250 million years ago,12,13 the earth lost 96% of all marine species, 100% of the coral reefs, and 70% of terrestrial vertebrates. In that event, the accumulation of carbon dioxide in the atmosphere led to ocean warming and to ocean acidification that together played a key role in the global loss of life. Recovery from that extinction event took more than 10 million years.

Currently, we are experiencing a 6th mass extinction,11 and we are approaching up to 100x higher rates of extinction than the background rate. There are two critical differences now. First, the current rate of change to the earth’s ecosystem is occurring in a few decades rather than over thousands of years as in the previous five extinction periods. Second, the events underlying the current cataclysm are man-made. Metaphorically, we are riding a runaway climate train with no one at the controls.

Effects on the Oceans

In the past there have been few established populations of invasive species identified in the high northern latitudes, that is, the northern coasts of Canada or Russia. With the continuing loss of Arctic sea ice, this situation will change. There has been rapid growth of shipping traffic along the northern coast of Russia in recent years, a large cruise ship went through the Northwest Passage in 2016, and now multiple arctic cruises are advertised each year. We can expect continuing expansion in arctic shipping activities, mineral/energy exploration, fishing, and tourism in future years. These new northern transport routes offer shorter and less expensive connections between northern hemisphere ports, so the shipping traffic will inevitably grow as more ice melts and warmer weather seasons get longer. Introduction of invasive species into these Arctic regions will follow rapidly. This will bring new challenges to the native inhabitants—humans, wildlife, and plants—of these northern ocean and terrestrial habitats. There will be greater competition for food sources and introduction of new infectious diseases. This sequence of events has occurred innumerable times before when alien populations expanded into new regions.14

Currently, the oceans absorb 93% of the heat trapped by greenhouse gases in the atmosphere, thus slowing warming of land masses. But the resulting rapid warming of the oceans directly impacts marine life and related food chains. Consider, for example, the coral reefs along over 93,000 miles of coastline rimming the oceans—one of the largest ecosystems on the planet.

A thriving coral reef is comprised of groups of millions of identical tiny polyps a few millimeters wide and a few centimeters long, each with a calcite skeleton. Millions of these tiny stony skeletons accumulate over generations to form the large hard coral reefs found along tropical shorelines. Many of the coral species obtain most of their nutrients from photosynthetic algae plants called zooxanthellae. When the sea around them warms excessively, the polyps expel the zooxanthellae and the coral becomes completely white—a condition called coral bleaching. Corals can survive bleaching events and restore the zooxanthellae, if conditions normalize quickly enough. But the bleaching events are highly stressful, and the corals will die if occurrence of bleaching events persists. When this happens, only the dead coral skeletons—which can be immense—are left.

The Great Barrier Reef, 500 feet thick at some points, extends discontinuously for over 1500 miles off the coast of eastern Australia. By 2018, half of the Great Barrier Reef had died from heat stress. Similar damage is occurring in the Caribbean and the rest of the world’s tropical shorelines.15,16

Loss of the ocean reef ecosystems could substantially compromise the Earths ability to sustain the health and well-being of its inhabitants. Fish populations in the coral reefs are the source of food for hundreds of millions of people. Loss of the reefs disrupts the marine food chain which causes loss of local food supplies, stressed populations, and conflicts over fishing rights.

There is now a global sense of urgency to develop methods to restore and maintain the health of the reefs considering their increasing destruction. Corals can evolve to survive in changed conditions—warmer, more acidic, etc. However, the rate of natural adaptation is too slow relative to the current rate of changes in their ocean environment, so there is widespread devastation of established reefs. This has led to efforts to accelerate the rate of adaptation. In some stressed reefs, small coral colonies are found that have successfully adapted to the local changes in temperature and increased acidity. Reef preservationists have shown that corals harvested from these colonies can be nurtured in coral “farms” and then used to seed new growth in damaged areas. Scientists are also experimenting with selective breeding to develop coral strains better adapted to changed conditions.17–19

In Indonesia another attempt at coral reef remediation involves attaching optimized coral polyps to metal rods planted within the compromised reefs. The application of a mild electric shock causes minerals in the water to precipitate and adhere to the metal structures, thus stimulating calcification with the goal of creating the more native ‘cement’ of a reef’s exoskeleton, referred to as ‘Biorock.’20 The resulting limestone surface increases the growth of the corals under conditions that would normally lead to their death. All these schemes are highly promising, but there are daunting cost and logistical barriers to scaling restoration efforts to address the vast areas of lost reefs.

Global Warming Is Changing the Distribution of Animal and Plant Pathogens

The last century has seen radical changes in the pattern, volume, and speed of transport of people and cargo between widely separated regions on the planet. One consequence has been the increase in direct long-distance human transport of dangerous infectious diseases by person to person transmission. Surveillance of travelers at entry points, coupled with identification, treatment, and when necessary, quarantine of the infected persons and their contacts, has been the response strategy. But diseases that are carried by intermediate vectors, for example, mosquitoes or ticks, present a different and more complex challenge. Any such vector is adapted to thrive in some environmental niche—characterized by a temperature and rainfall range, urban or rural, indoor or outdoor, etc. When a region’s climate warms, it may become hospitable to new vectors, which will then inevitably arrive either by expansion from adjacent territories or as accidental hitchhikers in freight shipments or transport vehicles.

For example, in a remarkably short time, human viruses like Zika, Dengue, Chikungunya, Yellow Fever, and West Nile have spread into regions of the Caribbean, Latin America, and the United States that until recently had ambient temperatures below that required to support their transmission. In addition, fungal infections of food plants, like the blights infecting Cavendish bananas and cocoa trees, have become a global problem. The rapid spread of global disease caused by changes in atmospheric temperature, ocean temperature, erratic and drenching rains, and floods in one geographic location accompanied by droughts in another location is being facilitated by migration of the vectors, such as mosquitoes, ticks, bats, and rats, that carry the pathogens. Insect vectors are exquisitely sensitive to changes in temperature, and warmer temperatures increase their breeding season and life span. Zika, Dengue, Chikungunya, and Yellow Fever viruses soon follow arrival of the common Aedes aegypti mosquito and are then transmitted among humans by the female mosquito. Other mosquito species transmit West Nile virus, the malaria parasite, and the parasitic nematode worm that causes the human disfiguring disease lymphatic filariasis (elephantiasis).

Ticks are another rapidly spreading vector. Although most tick species do not harbor pathogens harmful to humans, Lyme disease is caused by a tick-borne bacterial pathogen, Borrelia burgdorferi. Until recently, ticks were inhibited over much of North America by cold winters, but with increasing average temperatures and milder winters they are becoming established further north. Lyme disease is now endemic in Canada, so the government has recently established tick surveillance networks.

The vector-borne bacterial pathogen Candidatus Liberibacter that causes citrus greening disease is a serious agricultural threat. Liberibacter are transferred to citrus trees by an insect vector, the Asian citrus psyllid or jumping plant lice. The disease causes the decline and death of citrus trees by blocking the flow of nutrients and sugars from the leaves to the roots. Once infected, the tree is doomed. Liberibacter have recently migrated along with the citrus psyllid vector to warming temperate climate zones worldwide, including ten U.S. states.21 The resulting Citrus Greening infections have devastated the Florida citrus industry and destroyed citrus groves in Asia, Brazil, and the Dominican Republic. In the United States, the damage has been less in states further north than Florida, probably because of their cooler temperatures, but as the climate warms, the citrus greening infections will likely continue moving northward.

Owing to the huge financial impact of citrus greening, there are multiple biology-based efforts underway to disrupt the infection pathway either by eliminating the psyllid vector, by killing the bacterial Liberibacter pathogen, or by developing an infection resistant citrus tree variety.22 Insect warfare has also been tried by introduction of a wasp that preys specifically on the Asian citrus psyllid. This strategy works, but it only reduces, rather than eliminating, the citrus psyllid population.23

Each biological approach tried so far has its pros and cons. Insecticides can kill the citrus psyllid, but they may also threaten beneficial insects. Antibiotics may kill the Liberibacter, but their use can also increase bacterial antibiotic resistance and thus loss of antibiotic effectiveness for treating human diseases. This story of the challenges of containing the spread of the citrus greening disease is representative of similar challenges encountered in trying to deal with a myriad of newly encroaching diseases, some carried by other insect vectors. Are there better solutions on the horizon? It may be that recent advances in genetic technology will lead to more effective approaches.

Can New Genetic Technologies Reduce Global Warming Consequences?

Along with the increasing threat of climate change to human health and agriculture, we are experiencing a revolution in genetic engineering technology. Perhaps this will lead to new methods for effective surveillance and for mitigation of the redistribution of vectors that transmit disease.

The new CRISPR Cas9 technology lets us change specific genes in an insect or animal vector, thus making it either unable to serve as a reservoir for a given pathogen (known as a population modification drive) or eliminating the ability of the vector to propagate (known as a suppression drive). A suppression drive targets the reproductive capacity of the insect vector and can lead to a population crash, potentially wiping out a species. A population modification drive does not affect the reproduction capability of the insect, but it prevents the vector from harboring the pathogen or it prevents transmitting the pathogen to the human host. With these technologies, the genetic makeup of a few individuals in a targeted vector species is changed in such a manner that once these individuals are released into the wild, the change spreads rapidly throughout the entire vector population. Gene drives only affect sexually reproducing species, and thus they cannot be used directly on bacterial and viral pathogens.

Malaria transmission has been used as a test case to explore use of a vector gene drive to contain the spread of a disease. The results have been encouraging. In 2015, 200 million people worldwide were infected with malaria and between 500,000 and 700,000 died from the disease. Seventy-two percent of these were children under 5 years of age. In 2016, the number of cases worldwide increased to 216 million. Of 3,500 mosquito species, only those that belong to a subset called Anopheles can transmit the malaria parasite, Plasmodium falciparum, to a human by means of a bite from a female. The Anopheles stephensi mosquito, endemic to India and South Asia, carries the malaria parasite in that region. These mosquitoes were experimentally gene edited so that they could no longer carry the malaria parasite, establishing a population modification gene drive. A key trick in a gene drive is to engineer both copies of the chromosome so that all the offspring of a mating between a normal mosquito and a genetically altered one carry the genetic profile of the desired alteration, rather than just half the offspring, which is normally the case. Under laboratory conditions, it was demonstrated that this population modification drive leads to rapid spread of the desired genetically-altered mosquito and disappearance of the normal mosquitoes. The genetically altered mosquitoes cannot harbor the malaria parasite. This suggests that release of this genetically altered mosquito into the wild would halt the spread of malaria and thus save millions of lives. Eventually the malaria parasite could naturally mutate to overcome the genetic change in its mosquito host allowing it to once again infect humans, but this might not occur for a long time.

Another example is the Anopheles gambiae mosquito, which transmits malaria in sub-Saharan Africa. In another series of gene drive experiments, gene editing was used to change genes that the female mosquito needs for egg production, thereby creating female sterility (a suppression gene drive). In this case, the goal was just to reduce the number of mosquitoes transmitting malaria, but the technique could potentially wipe out the entire population of Anopheles gambiae. The combined challenge of climate change, which is altering the geographic distribution of the vector mosquitoes, and growing resistance to drugs routinely used to treat malaria-infected patients is making gene editing of the insect vectors an increasingly attractive potential solution. However, the notion of eliminating an entire insect species troubles many people.

In another test case, gene drives are being explored as a way of controlling transmission of Lyme disease by ticks on the U.S. island of Nantucket. Owing to recent increases in the population of island ticks, over 40% of the 10,000 inhabitants of Nantucket have, or have had, Lyme disease. Both deer and the white foot mouse can transmit the Lyme disease pathogen, Borrelia burgdorferi bacteria, to ticks, and the pathogen can then be transmitted to humans by the ticks. Ticks feed on the deer or white foot mice carrying Borrelia and the infected ticks bite humans, passing on Lyme disease. A plan was proposed by Kevin Esvelt (MIT) and Sam Telford (Tufts U., Cummings School of Veterinary Medicine) to use a gene drive to reduce the population of white footed mice that are infected with Borrelia. To do this, the mice would be genetically engineered so that they are immune to infection by the Lyme disease bacterial pathogen and thus could not accumulate infectious Borrelia. In this case, there would still be the same number of mice and the same number of ticks, but the number of ticks able to transmit Borrelia would be significantly reduced. Thousands of altered mice would be released on the island. The gene drive would ensure that the genetic alteration would pass down through all following generations of mice on the island, disrupting the cycle of transmission. The plan is to first test the genetically modified mice on an uninhabited island and then, with the concurrence of the inhabitants of both Nantucket Island and Martha’s Vineyard, release the genetically altered mice. The first step will be to get the concurrence and support of the inhabitants of these islands, because the gene drive would be altering the environment shared by all inhabitants.

Recently, a new gene editing application has been developed to alter the response of plants to environmental challenges. The proposed scheme involves spraying a field of plants with millions of insect vectors carrying viruses that are programmed to edit the genome of a plant such as maize to become drought resistant, in one growing season. This technique would be significantly faster than a gene drive. Further, this method would not permanently alter the genetic makeup of future plant generations, as is the case with gene drives. The goal is to engineer drought-resistant and temperature-tolerant plants, thereby securing the food supply during times of climate instability. But there is a catch, as once released into the wild, controlling these insect vectors would be difficult, if not impossible. As a result, this work has been limited so far to the laboratory. There is also concern that the method could be adapted as a biological weapon, enabling destruction of targeted food crops over wide areas by adverse genetic manipulation of the plants’ chromosomes. In addition to controlling mosquito vectors and tick-borne Lyme disease, gene drives are also being devised to control the nematode worms that carry the parasite causing Schistosomiasis.

Gene drives have not yet been released in the wild to mitigate vector-borne transmission of disease as there are critical questions to be resolved as noted above. Although the biology is ready, there are many questions of governance, safety, and ethics to be answered. Caution is important, since once the genetically-altered vectors are released, there is no assured way of controlling them at this point.

In July 2015, the U.S. National Academy of Sciences convened a meeting to discuss “the promise and perils of gene drives.” Critical questions raised at the meeting were:

Will an entire species of vector be wiped out? Methods are being devised to slow the gene drive so that only a portion of the offspring contain the genetically engineered alterations. These “Daisy chain drives,” have been engineered to be self-limiting and eventually disappear from the population.

Have techniques been devised that could control a runaway gene drive? By creating a second gene drive that undoes the genetic alterations of the first gene drive, essentially “a molecular eraser,” it is hoped a gene drive could be reversed, but not before unintended consequences to the ecosystem become apparent.

Can the altered genetic traits be transferred to other insect species? Unlikely, but possible. If this occurred, the potential for wiping out beneficial insect species would lead to further ecological disruptions, compounding the ravages of climate change.

Global Warming Mitigation Will Require a Coordinated International Effort

Many climate scientists and other thoughtful people have had concerns about the deteriorating global ecosystem for several decades now. The contribution of human activity to this escalating cataclysm is well documented. Predictions of dire consequences have been noted and sporadic attempts by the international community have been made to mitigate the ongoing onslaught of carbon emissions. But global warming is a problem that can only be solved by global cooperation because the world’s ecosystem is an integrated system. The causes of environmental degradation cannot be addressed by a patchwork of uncoordinated responses. We are dependent upon achieving international cooperation to mount a coordinated, science-based response.

In the United States today, political calculations relating to oil and coal interests have halted government acknowledgement of the risks of continuing future emissions of CO2 into the atmosphere. In December 2018, at a UN Climate Change Conference in Poland, Wells Griffith, Mr. Trump’s international energy and climate adviser, said “We strongly believe that no country should have to sacrifice their economic prosperity or energy security in pursuit of environmental sustainability.” The attendees broke into jeers and mocking laughter.24 Do not think that the United States is alone in this stance. We are aligned with other major fossil fuel producing nations, including Russia, Saudi Arabia, Kuwait, and Australia. We are now well beyond the time of debating about validity of the predictions about what will happen if climate change is left unaddressed. Rather, we are trying to mitigate what has already happened, while, as a society, summoning the courage and the will to leave fossil fuels in the ground and switch to alternative energy sources. Renewable power resources and improvements in the efficiency of our energy use can be important components of our energy future for the rest of this century. But, practically speaking, nuclear power will probably also have to be a major component of the future energy portfolio in order to meet world energy demands while greatly reducing use of fossil fuels.25, 26 That too is controversial. These are existential choices that call for an unprecedented level of wisdom and societal responsiveness in the world’s political systems. It does seem likely that achieving the necessary global political response will only come when there is widespread public fear and panic as the realization of the danger percolates into public consciousness.27 It is extraordinary that the current U.S. national leadership both denies existence of the global warming problem and actively promotes more use of fossil fuels. The longer we delay reduction in global CO2 emissions, the worse the ultimate catastrophe will be.

Authors’ Note:

We believe the world energy economy must shift rapidly from reliance on fossil fuels—coal, oil, and gas—to cleaner alternatives or our children and grandchildren will suffer dire consequences. We encourage the reader to personally assess the risks and potential solutions. To that end, we have included references for further reading that are openly accessible on the Internet.

 

Lucy Shapiro is a professor in the Department of Developmental Biology at Stanford University School of Medicine where she holds the Virginia and D. K. Ludwig Chair in Cancer Research and is the director of the Beckman Center for Molecular and Genetic Medicine. Harley McAdams is an emeritus professor at the Department of Developmental Biology at Stanford University School of Medicine.


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