MADRID — Delegates from nearly every nation spent the last two weeks here at a United Nations climate summit struggling to chart a course to meet the extraordinarily difficult goal of net zero emissions of carbon dioxide by the year 2050.

Yet long before then, the effects of global warming could spin out of control. As the United Nations’ secretary general, António Guterres, warned in opening the meeting: “The point of no return is no longer over the horizon. It is in sight and hurtling toward us.”

Perhaps nowhere is that more true than in the Arctic. The surface air there is warming at twice the global rate and temperatures over the past five years have exceeded all previous records since 1900. This past week, the National Oceanic and Atmospheric Administration reported that the extent of Arctic summer sea ice was at its second lowest point since satellite observations began in 1979, and that average temperatures for the year ending in September were the second highest since 1900, when record-keeping began.

What will this mean? A study published in Geophysical Research Letters in June described the catastrophic consequences of losing the Arctic’s reflective summer sea ice. The ice is a great white shield that reflects incoming solar warming back to space during the long summer days of the midnight sun. Otherwise, it would be absorbed by the ocean. Losing this ice, the study explained, would be the warming equivalent of an extra 25 years of emissions at current rates, pushing us more quickly past the threshold of warming that scientists say could lead to catastrophic damage, from more intense heat waves and coastal flooding to extinctions of species and threats to food supplies.