In 1992, over 1700 of the world's leading scientists came together to publish an article titled, "World Scientists' Warning to Humanity." These independent researchers, among them the majority of world's living Nobel Laureates in the sciences, authored the paper to introduce the public to the possibility of environmental catastrophe, should "business as usual" levels of environmental degradation continue. They hoped to inspire world leaders to take serious efforts to curb environmental degradation.

However, a second notice issued this past November states that in the quarter-century since the 1992 article was published, not enough action has been taken to stop environmental degradation. Similar to the 1992 warning, over 15,000 independent scientists from more than 180 countries came together in 2017 to publish this second warning to the world: "World Scientists' Warning to Humanity: A Second Notice."

The main issues cited by the original 1992 paper were the rapid depletion of atmospheric ozone (essentially, Earth's sunscreen) by chlorofluorocarbon (CFC) emissions, the overexploitation of resources in the planet's oceans and soils, deforestation, and the beginnings of what scientists are now calling the sixth mass extinction. To fix these problems, the leading global researchers of the 1990s described the need for "five inextricably linked areas [to be] addressed simultaneously."

The researchers recommended an end to human activities like greenhouse gas emissions, pollution, and deforestation, and more sustainable management of natural resources.

They also promoted the reduction of global poverty, lessening the environmental problems – such as unchecked pollution, poaching, and illegal logging – that frequently come alongside it.

Finally, the scientists called for stabilization of global population growth and for gender equality, especially in regards to women's reproductive rights. In 1992, global population was estimated to peak between 12.5 and 14 billion people – a number far greater than researchers hypothesized the world's resources could support. To curb this growth, scientists called for women to have full control over their reproductive decisions, as well as better access to education and family planning. These resources have lowered birth rates in developing countries and could help reduce stress on the Earth's ecosystems.

Despite these pieces of advice, however, in the quarter of a century since scientists published the warning, and the suggestions for how to best heed it, only one piece of these larger environmental issues has been fully addressed: limiting the harmful CFC emissions that created a hole in the ozone layer in the late 1900s.

CFCs are chemical compounds that were a common ingredient refrigerants and aerosols throughout the late 20th century. Though they are largely considered harmless at ground level, they cause significant damage when emitted into the atmosphere. They react with ozone molecules to create oxygen and chlorine monoxide, which reacts with ozone alongside CFCs to break up more ozone molecules. Ozone is necessary for protection from the sun's powerful rays, and when CFCs are released into the atmosphere, they eventually form a hole in the ozone layer.

The discovery of this issue prompted international response, culminating in the 1987 Montreal Protocol, which banned CFCs in the 46 signatory countries. Today, all of the world's 197 governing bodies have ratified the plan.

Despite the success of the Montreal Protocol, the other problems outlined in the original warning have only worsened. This lack of success is what prompted scientists, on the 25th anniversary of the original article, to publish "A Second Notice" to humanity.

One of the authors of this second warning, environmental science and ethics expert Dr. Eileen Crist, says the issue of CFCs creating an ozone hole was more easily fixed by the international community than other forms of atmospheric pollution, like greenhouse gas emissions.

"Banning CFCs was not so hard. CFCs were coming out of a particular sector of the economy and…at the moment where they were looking [to phase] out CFCs, there was a readily available alternative."

Crist maintains that banning CFCs pales in comparison to the issue of stopping climate change – a direct result of greenhouse gas emissions from burning fossil fuels.

"The entire global economy – both in terms of energy use and materials use – is utterly beholden to fossil fuels," Crist said. "All you have to do is look around you: your environment [is] full of plastic and it's full of energy that is using fossil fuels… it's a much bigger problem to solve."

Though "A Second Notice" does include a list of suggestions to tackle the planet's prevailing environmental issues, the first half of this paper serves as comprehensive summary of the continued failures (and single success) of the past 25 years: the steady increase in greenhouse gas emissions and human population, and the staggering drop in wildlife abundance and forest cover.

Just as the original paper warned humankind that "a great change in our stewardship of the earth and the life on it is required" to prevent environmental catastrophe, this year's follow-up warning cautioned, "Soon it will be too late to shift course away from our failing trajectory, and time is running out."

In order to truly solve the environmental problems wracking the planet, Crist maintains, humankind must be proactive rather than reactive: instead of focusing on ways to fix present and future damage, people need to focus on stopping the destruction right now. Dr. Mohammed Alamgir, an expert in ecosystem services and another co-author of "A Second Notice," agrees.

"We need to change our own behavior when it comes to reproduction, when it comes to the consumption of foods…[and] power," Alamgir said. "We have to work together…politicians can't fix everything. They should play the major role, but individuals have a responsibility."
Despite the fact that, in most respects, the world ignored scientists' first warning 25 years ago, Alamgir remains optimistic about the future of the planet.

"If we look back at the first warning in 1992, after 25 years we definitely didn't do enough, but it's not true that we didn't do anything at all…We had success in [fixing] ozone layer depletion…in decreasing the reproduction in some parts of the country, to reducing deforestation in some parts of the world and reducing poverty," Alamgir insisted. "Definitely, we are hopeful."

Researchers suggest that destruction can be mitigated by protecting wilderness areas, rewilding regions with apex predators, promoting dietary shifts towards mostly plant-based foods, and more – and by calling for the world's governing institutions to do the same.

In the article's conclusion, Alamgir, Crist, their six fellow co-authors, and their over 15,000 scientific peers note, "We must recognize, in our day-to-day lives and in our governing institutions, that Earth with all its life is our only home."