Is the world's climate changing? From all over the world this year come reports of oddities: too much rain - too little rain; mud slides - altered ocean currents; excessive heat - unusual cold. All the time there are social changes on Earth, too: more people to feed and a world threatened with overcrowding. Are the two connected?
I listened to Jimmy Carter address the Global Tomorrow Coalition conference here last week, a group with perhaps 6 million members. It was reassuring to see people solicitous about the state of the Earth. That's where the question of climate came in. ''Carbon dioxide concentrations in the upper atmosphere are already warming the earth and melting the polar icecaps,'' he said. ''Acid precipitation is sterilizing once fertile lakes and streams and seriously reducing productivity of farmland.''
There is widespread dispute over this one: One school of thought ridicules warnings of doomsday. As to food, they argue, the new miracle seed is meeting the problem; we should protect the environment, of course, but the crisis is in the future. That doesn't satisfy the environmentalists. Take fuel, for example; they say coal is replacing oil, contributing to a buildup of carbon dioxide ''which may lead to a warming of the climate, altered patterns of rainfall and snow, disruption of agriculture, and the possibility of melting polar icecaps, thus raising sea levels.''
(I have a guilty feeling that I have been neglecting those polar icecaps; after listening to dedicated environmentalists a couple of days I have a feeling that we'll be fishing out of the Empire State Building's fifth story before long.)
This is no time for flippancy, however. Global population is racing ahead. Back in the Stone Age (8000 BC) Earth's population was hardly visible; in the Middle Ages, some nations began to feel crowded. But today population growth is hard to appreciate: Earth's total increase by one estimate will leap from an estimated 4 billion in 1975 to 6 billion in 2000. Now the rate of increase is slowing. Food production is keeping pace with the increase in people, demographers say, thanks chiefly to better seeds, better fertilizer, and better methods. As one study put it to the conference here: ''There is enough or could be enough food to feed even the 10.5 billion forecast as the ultimate size of world population when it stabilizes in 2110 AD (the United Nations medium projection).''
Jimmy Carter appeared at last week's conference with a ruddy tan and a smile: amazed at the price of food. When he ''left Carter's Warehouse in Plains, Ga.,'' he said, he was selling a ton of fertilizer for less than $40; ''now the same product sells for more than $125.'' He was anxious and urgent. He recalled that under his presidency he authorized what became known as the Global 2000 Report: It studied the world's limited resources. He reproached the Reagan administration for not following it up: ''The needs we identified before January 1981 are now much more acute,'' he declared. He sharply attacked what he called ''the deliberate across-the-board abandonment of US leadership on environmental, resource, and related global issues.''
The Carter report began with a now famous sentence: ''If the present trends continue, the world in 2000 will be more crowded, more polluted, less stable ecologically, and more vulnerable to disruption than the world we live in now.'' But such things don't have to happen, Mr. Carter insisted. He gave a kind of inventory of the world situation as he sees it. On Spaceship Earth ''hungry mouths are increasing at least as fast as food supplies,'' he warned. He said that because of excessive fishing and high fuel costs the total worldwide fish catch has leveled off. He said ''the most acute pressures'' are on renewable sources, croplands, forests, fisheries, and water. He declared that ''the Earth's 2 billion acres of desert will increase by 20 percent during the next two decades.'' In this single year, Mr. Carter said, ''there are about 90 million new people to feed; by year 2000 each acre of arable land will have to support one-third more people. In some parts of the world, like Africa, food production per person is already showing a steady decline.''
Mr. Carter's declaration was measured and solemn. He addressed a special audience already convinced but clearly not satisfied with what the Reagan government is doing. Mr. Carter did not seem to be making a partisan attack; it appeared that he was looking back on his White House incumbency and finding satisfaction in his leadership in this field. He supplied detail. He said that the World Bank estimates that ''absolute poverty'' now afflicts perhaps a billion people and is likely to grow bigger.
''We must realize,'' he said, ''that without leadership from within our own country concerted action among other nations is unlikely.'' Such leadership, he charged, flatly hasn't been forthcoming. He added, ''The needs we identified before January 1981 are now much more acute.''