Imagine there are no people. Imagine a planet where the sea level is about five to 40 meters (16 to 131 feet) higher than normal. Imagine a planet that is hotter and wetter. Imagine, worldwide, it’s roughly 3 to 4 degrees Celsius (5.4 to 7.2 degrees Fahrenheit) warmer than today. And the North and South poles are even warmer still – as much as 10 degrees Celsius (18 degrees Fahrenheit) hotter than today.
Welcome to the Pliocene. That was the Earth about three to five million years ago, very different to the Earth we inhabit now. But in at least one respect it was rather similar. This is the last time that carbon dioxide (CO2) levels were as high as they are today.
On May 9, 2013, CO2 levels in the air reached the level of 400 parts per million (ppm). This is the first time in human history that this milestone has been passed.
CO2 is the most important man-made greenhouse gas, which means (in a simple sense) that it acts like a blanket trapping heat near the surface of the Earth. It comes from the burning of fossil fuels such as coal, oil and natural gas, as well as deforestation. The level of CO2 in the atmosphere has risen from around 317 ppm in 1958 (when Charles David Keeling began making his historical measurements at Mauna Loa) to 400 ppm today. It’s projected to reach 450 ppm by the year 2040.
To some, crossing the threshold of 400 ppm is a signal that we are now firmly seated in the “Anthropocene,” a human epoch where people are having major and lasting impacts on the planet. Because of the long lifetime of CO2, to others it means we are marching inexorably towards a “point of no return,” into territory that is unknown for the human race.
Each year, four international science institutions compile temperature data from thousands of stations around the world and make independent judgments about whether the year was warmer or cooler than average. “The official records vary slightly because of subtle differences in the way we analyze the data,” said Reto Ruedy, climate scientist at NASA’s Goddard Institute for Space Studies. “But they also agree extraordinarily well.”
All four records show peaks and valleys in sync with each other. All show rapid warming in the past few decades. All show the last decade has been the warmest on record.
You can also find this graphic on NASA’s Climate 365 Tumblr page.
Many climate scientists agree that significant societal, economic, and ecological damage would result if global average temperatures rose by more than 2 °C (3.6 °F) in such a short time. Such damage would include increased extinction of many plant and animal species, shifts in patterns of agriculture, and rising sea levels. By 2015 all but a few national governments had begun the process of instituting carbon reduction plans as part of the Paris Agreement, a treaty designed to help countries keep global warming to 1.5 °C (2.7 °F) above preindustrial levels in order to avoid the worst of the predicted effects. Authors of a special report published by the IPCC in 2018 noted that should carbon emissions continue at their present rate, the increase in average near-surface air temperatures would reach 1.5 °C sometime between 2030 and 2052. Past IPCC assessments reported that the global average sea level rose by some 19–21 cm (7.5–8.3 inches) between 1901 and 2010 and that sea levels rose faster in the second half of the 20th century than in the first half. It also predicted, again depending on a wide range of scenarios, that the global average sea level would rise 26–77 cm (10.2–30.3 inches) relative to the 1986–2005 average by 2100 for global warming of 1.5 °C, an average of 10 cm (3.9 inches) less than what would be expected if warming rose to 2 °C (3.6 °F) above preindustrial levels.
Of all these gases, carbon dioxide is the most important, both for its role in the greenhouse effect and for its role in the human economy. It has been estimated that, at the beginning of the industrial age in the mid-18th century, carbon dioxide concentrations in the atmosphere were roughly 280 parts per million (ppm). By the middle of 2018 they had risen to 406 ppm, and, if fossil fuels continue to be burned at current rates, they are projected to reach 550 ppm by the mid-21st century—essentially, a doubling of carbon dioxide concentrations in 300 years.
A vigorous debate is in progress over the extent and seriousness of rising surface temperatures, the effects of past and future warming on human life, and the need for action to reduce future warming and deal with its consequences. This article provides an overview of the scientific background and public policy debate related to the subject of global warming. It considers the causes of rising near-surface air temperatures, the influencing factors, the process of climate research and forecasting, the possible ecological and social impacts of rising temperatures, and the public policy developments since the mid-20th century. For a detailed description of Earth’s climate, its processes, and the responses of living things to its changing nature, see climate. For additional background on how Earth’s climate has changed throughout geologic time, see climatic variation and change. For a full description of Earth’s gaseous envelope, within which climate change and global warming occur, see atmosphere.
Global warming is related to the more general phenomenon of climate change, which refers to changes in the totality of attributes that define climate. In addition to changes in air temperature, climate change involves changes to precipitation patterns, winds, ocean currents, and other measures of Earth’s climate. Normally, climate change can be viewed as the combination of various natural forces occurring over diverse timescales. Since the advent of human civilization, climate change has involved an “anthropogenic,” or exclusively human-caused, element, and this anthropogenic element has become more important in the industrial period of the past two centuries. The term global warming is used specifically to refer to any warming of near-surface air during the past two centuries that can be traced to anthropogenic causes.
Boosting seedling production and planting them means increasing support and investment across the entire process. As the study found, there has been “chronic under-investment” in specialized labor, infrastructure, and training. “Workforce challenges,” said Sprague, “are the number one barrier to scaling up.”
Seed collectors need to understand everything from predicting when certain species will release their seeds—making them available to gather—to how to safely clean seeds. Staff then need to be trained on how to test the seeds’ quality and store them so they stay viable over the years. “It’s a perishable product; it needs to be treated carefully,” said study co-author Greg Edge, a forest ecologist with the Wisconsin Department of Natural Resources’ Forestry Division. Yet the number of people specializing in this work continues to dwindle.
Nurseries, meanwhile, rely only on a handful of year-round staff; the rest are seasonal workers who help with sowing, harvesting, sorting, and packing. It can be difficult to attract these workers, though, due to the remote locations of many nurseries, as well as competition from other agriculture jobs. Immigration policies can also affect the number of available workers, the study noted.
The global response to the COVID-19 pandemic has led to a sudden reduction of both GHG emissions and air pollutants. Here, using national mobility data, we estimate global emission reductions for ten species during the period February to June 2020. We estimate that global NOx emissions declined by as much as 30% in April, contributing a short-term cooling since the start of the year. This cooling trend is offset by ~20% reduction in global SO2 emissions that weakens the aerosol cooling effect, causing short-term warming. As a result, we estimate that the direct effect of the pandemic-driven response will be negligible, with a cooling of around 0.01 ± 0.005 °C by 2030 compared to a baseline scenario that follows current national policies. In contrast, with an economic recovery tilted towards green stimulus and reductions in fossil fuel investments, it is possible to avoid future warming of 0.3 °C by 2050.
By the time the World Health Organization declared COVID-19 (scientifically referred to as the severe acute respiratory syndrome–coronavirus 2 or SARS-CoV-2) a pandemic on 11 March 2020, the virus had already spread from China to other Asian countries, Europe and the United States. As of 5 July 2020, cases have been identified in 188 countries or regions1. This has led to unprecedented enforced and voluntary restrictions on travel and work. This in turn has led to reductions of both GHG emissions and air pollutants2,3,4. Analysis of mobility data from Google5 and Apple6 shows that mobility declined by 10% or more during April 2020 in all but one of the 125 nations tracked. Mobility declined by 80% in five or more nations (Supplementary Fig. 1). Associated declines in air pollution have been observed from satellite data and from local ground-based observations7,8. The large pollution declines are expected to be temporary as pollution levels are already returning to near-normal in parts of Asia9,10.
Here we build an estimate of emission changes in GHGs and air pollution due to the COVID-19 global restrictions during the period February–June 2020 and project these into the future. These emission changes are then used to make a prediction of the resultant global temperature response. We examine the temperature response of a direct recovery to pre-COVID-19 national policies and emission levels, and also explore responses where the economic recovery to COVID-19 is driven by either a green stimulus package or an increase in fossil fuel use.
Planting more trees to offset carbon emissions will further increase demand for seedlings. The good news, said Fargione, is that only a third of public and private nurseries surveyed are currently operating at full capacity. That means there’s a big opportunity to expand.
Seedling production peaked over 30 years ago. In the late 1980s, more than 2.6 billion seedlings were produced each year in the U.S. Once the 2008 recession shuttered many nurseries across the country, that number dropped to less than one billion. “Imagine losing 75 percent of your capacity,” said Dan Rider, associate director of the Maryland Forest Service, of the impact that the recession and other factors have had on the state’s John S. Ayton forest tree nursery. “Our story’s not unique.”
It’s going to take a lot of work to scale back up, said Eric Sprague, vice president of reforestation at American Forests, a conservation organization that helped spearhead the new study as well as the trillion-trees initiative. But doing so, he says, is “going to be a huge part” of whether or not the U.S. achieves its reforestation goals.
“It’s not just about expanding and improving what we have,” Sprague said, “but actually adding new nurseries to meet this goal.”
If all nurseries, both public and private, operated at maximum capacity, the study estimates an additional 400 million more seedlings could be grown each year. Researchers also expect a further 1.1 billion seedlings could be produced annually if the majority of nurseries expanded beyond their current capacity, which most surveyed in the study said they’d be willing to do. Add all of this to the 1.3 billion currently being grown, and production would be nearly at the three billion per year minimum that the study recommends.
Bottom-up emission-trend analyses have traditionally relied on laborious collection of various energy-industry-related indicators and statistics from multiple sources. The unprecedented recent access to global mobility data from Google and Apple gives a unique opportunity to compare trends across many countries with a consistent approach. We use these data to develop a new method of emission-trend analysis. The advantage over previous approaches is the possibility of near-real-time analysis, national granularity and a systematic consistent approach across nations and over time. The disadvantages are the loss of a direct connection between energy and emissions and the need to make assumptions about these relationships. There are also disadvantages over the short time history of the mobility data and opacity from the data providers around their detailed methodologies and uncertainties. Here we make a simple set of assumptions to deduce estimates of emissions change from the mobility data and test the estimates extensively against the approach of Le Quéré et al.
Google and Apple mobility changes and the Le Quéré et al. data all indicate that >50% of the world’s population reduced travel by >50% during April 2020. Google mobility trends indicate that >80% of the population in the 114 countries in the dataset (4 billion people) reduced their travel by >50%. Google mobility data and emission reduction estimates based on confinement level analysis in Le Quéré et al. agree on country-level surface-transport trends to within ~20%. When we examine the trends for the countries that we expect have contributed most to the overall surface-transport emission change (for example, the United States, European nations and India), good agreement between the datasets is observed and their trends are well-correlated in time.
Pressure to regrow and protect forests is at an all-time high. Last August, over two dozen local governments, companies, and nonprofits across the U.S. committed to the World Economic Forum’s initiative to globally plant a trillion trees by 2030. Last October, then-President Donald Trump signed an executive order committing the U.S. to that same goal. With such bipartisan support, some environmental nonprofits are hopeful the Biden administration will build on this initiative.
Beyond individual corporate or local goals, growing trees can contribute something to national climate targets. The land sector—which includes everything from tree planting and avoiding deforestation to increasing the amount of carbon stored in soils—accounted for a small portion of the U.S. Obama-era commitment to reduce emissions by up to 28 percent under the Paris Agreement. According to one estimate, if all 64 million acres identified in the study were reforested, that would represent roughly 7.5 percent of the emission reductions needed to meet the nation’s Paris Agreement commitments.
The current rate of reforestation, however, can’t even keep up with the amount of land that has been burned by devastating wildfires across the American West in recent years. Climate change is only expected to make wildfires more intense, which will increase that backlog.
“We’re just now recognizing the increasing backlog of areas that need to be planted that aren’t being met yet,” said seed ecologist and study co-author Olga Kildisheva, a project manager at The Nature Conservancy.
And even a tree-planting campaign can be doomed by a “misplaced emphasis on how many trees are planted rather than how many survive,” the study warned. It calls for developing guidelines on what seeds will thrive in different environments, especially as climate change shifts plant species to new regions.
“It’s not just about planting a tree. It needs to be done thoughtfully and well, because you can’t just stick a tree in the ground and come back in 100 years and have a forest,” said Edge. It takes an immense amount of money, labor, and patience to turn a seed into a sapling. “We don’t want to just waste our time sticking a seedling in the ground that’ll die.”