"Scientists have a moral obligation to clearly warn humanity of any catastrophic threat and to 'tell it like it is,'" a group of scientists wrote in a letter published in BioScience Tuesday. "On the basis of this obligation and the graphical indicators presented below, we declare, with more than 11,000 scientist signatories from around the world, clearly and unequivocally that planet Earth is facing a climate emergency."

The letter was written by dozens of scientists and endorsed by thousands more from 153 countries. It differs from past scientific communications about the climate crisis in that it does not speak in uncertainties and it lays out actual policy recommendations, The Washington Post pointed out. It is also the first time a large number of scientists have embraced the word "emergency" to describe climate change.

"The term 'climate emergency' … I must say, I find it refreshing, really," climate scientist and Woods Hole Research Center President Phil Duffy, who signed the letter Monday, told The Washington Post. "Because you know, I get so impatient with the scientists who just are always just waffling and mumbling about uncertainty, blah, blah, blah, and this certainly is, you know, is much bolder than that," he said. "I think it's right to do that."

Lead author and Oregon State University professor William Ripple told The Guardian he was inspired to spearhead the effort because of the uptick in extreme weather events he had noticed.

"It is more important than ever that we speak out, based on evidence. It is time to go beyond just research and publishing, and to go directly to the citizens and policymakers," he told The Guardian.

The letter writers outlined several "vital signs" of both human activities that contribute to global warming and climate impacts themselves.

"Policymakers and the public now urgently need access to a set of indicators that convey the effects of human activities on GHG emissions and the consequent impacts on climate, our environment, and society," they wrote.

Their indicators for human activities are

1. Human population
2. Total fertility rate
3. Ruminant livestock population
4. Per capita meat production
5. World Gross Domestic Product (GDP)
6. Global tree cover loss
7. Brazilian Amazon rainforest loss
8. Energy Consumption
9. Airplane passengers per year
10. Total institutional assets divested from fossil fuels
11. Carbon dioxide emissions
12. Carbon dioxide emissions per capita
13. Greenhouse gas emissions covered by carbon pricing
14. Carbon price per tonne of carbon dioxide
15. Fossil fuel subsidies

For the scientists, the most worrying indicators were the increases in human and livestock population, meat production, world GDP, tree cover loss, fossil fuel consumption, number of airplane passengers and carbon dioxide emissions both generally and per capita.

Positive signs included decreases in the fertility rate and Amazon deforestation and increases in wind and solar power consumption, divestment from fossil fuels and the proportion of emissions covered by carbon pricing.

"However," they noted, "the decline in human fertility rates has substantially slowed during the last 20 years, and the pace of forest loss in Brazil's Amazon has now started to increase again. Consumption of solar and wind energy has increased 373% per decade, but in 2018, it was still 28 times smaller than fossil fuel consumption. As of 2018, approximately 14.0% of global GHG emissions were covered by carbon pricing, but the global emissions-weighted average price per tonne of carbon dioxide was only around US$15.25. A much higher carbon fee price is needed."

The signs the scientists chose to monitor the impacts of the climate crisis were

1. Carbon dioxide in the atmosphere
2. Methane in the atmosphere
3. Nitrous oxide in the atmosphere
4. Surface temperature
5. Minimum Arctic sea ice
6. Greenland ice mass change
7. Antarctic ice mass change
8. Glacier thickness change
9. Ocean heat content change
10. Ocean acidity
11. Sea level change
12. Area burned in the U.S.
13. Extreme weather events
14. Annual losses due to extreme weather events

The letter writers warned of "untold suffering" unless measures were taken to address the climate crisis. They proposed six.

1. Energy: They called for a rapid shift to renewable energy and for leaving fossil fuels in the ground. They also called for negative emissions technology to remove carbon dioxide from the atmosphere, and for wealthy countries to help poorer countries pay for the transition.
2. Short-lived pollutants: They called for a reduction in shorter-lived pollutants like methane, black carbon and hydrofluorocarbons, which could reduce short-term warming by more than 50 percent and have the added benefit of saving lives from air pollution.
3. Nature: They called for preserving and restoring ecosystems like forests and coral reefs. They said that as much as one third of the emissions reductions needed by 2030 to achieve the Paris agreement goals could be met by preserving and planting natural carbon stores.
4. Food: They called for a shift towards a mostly plant-based diet and a decrease in meat and dairy consumption, which would decrease emissions and free up agricultural land for human food and ecosystem restoration.
5. Economy: They called for a carbon-free economy that did not rely on excessive resource extraction. "Our goals need to shift from GDP growth and the pursuit of affluence toward sustaining ecosystems and improving human well-being by prioritizing basic needs and reducing inequality," they wrote.
6. Population: They also wrote of the need to stabilize and, if possible, reduce the world population. They argued that there were ways to do so that also encouraged human rights, such as making access to family planning available to all and achieving gender equality, especially by encouraging education for young women and girls.

Attempts to limit or control population growth have proved a sensitive topic in the past, so much so that the UN does not negotiate on it, BBC News reported. But the writers thought it was important to address.

"It is certainly a controversial topic ― but I think that population should be talked about when considering human impacts on the earth," letter author Dr. Thomas Newsome of the University of Sydney said.