The report concludes that “global climate continues to change rapidly compared to the pace of the natural variations in climate that have occurred throughout Earth’s history.” It finds that “human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century.” The bottom line is that this report confirms and strengthens what the vast majority of climate scientists have known for decades: that climate is changing and humans are primarily responsible.

Recent observations and investment in modeling and research have only strengthened the quality and amount of evidence collected. As the report documents, each of the last three years has successively been the warmest on record based on observational data going back to the late 19th century, and 16 of the last 17 years have been among the 17 warmest years on record globally. Global sea level has risen by about 7 to 8 inches since 1900, with nearly half this rise occurring since 1993. A substantial component of this rise, which is accelerating the increased frequency of disruptive “nuisance” flooding in dozens of coastal American cities, is because of human activity. At the same time, the area of ocean covered by Arctic sea ice in September (the typical annual minimum) has decreased by about 50 percent, while its volume has decreased even more dramatically as the remaining ice thins.

The report also highlights growing reasons for concern. For example, ocean acidification, which occurs when atmospheric carbon dioxide is absorbed by seawater, is taking place at what is thought to be the fastest rate in at least 66 million years. Coupled with reductions in oxygen content in near-coastal American waters, this poses a significant threat to coastal fisheries and ecosystems. Much of the western United States is facing a growing threat of more severe drought and larger wildfires as higher temperatures, reduced snow pack and earlier spring snow melt reduce water availability during the warm season.

To stabilize global temperature, net carbon dioxide emissions must be reduced to zero. The window of time is rapidly closing to reduce emissions and limit warming to no more than 3.6 degrees Fahrenheit or 2 degrees Celsius above preindustrial levels, the goal set in the Paris climate accord. The further we push the climate system beyond historical conditions, the greater the risks of potentially unforeseen and even catastrophic changes to the climate — so every reduction in emissions helps.

While climate models incorporate many important processes, they cannot include all aspects of the climate system and all of the possible interactions within that system. Vicious cycles between these climate components may push the Earth into states much different from the past: for example, one with a much smaller West Antarctic Ice Sheet and much higher sea level, or one without coral reefs and with greatly reduced marine biodiversity. Surprises can also come from compound extreme events like droughts, floods, heat waves, hurricanes and wildfires that may occur in multiple places at the same time, or sequentially in one place. What is clear is that, even though we cannot quantify all of the possible changes to every element of the climate system, the risks to things we care about — from the health of our children, to the future economic viability of our low-lying coastal cities and infrastructure — are real and growing.