It’s been a strange year in Harlem. The robins didn’t migrate. The hyacinths popped up in the snow. And then tropical intruders took over the garden. This global warming is getting out of control.

It’s not that it happened all of a sudden. For some time I had noticed climatic changes in the backyard of our New York brownstone. The forsythia had not flowered for several springs. Each autumn we spotted fewer bird species flying south. And then there was that hothouse orchid that someone planted outside. It came back the following summer!

But, being a relative novice at gardening,I didn’t think much of these shifts in weather – at least not until one historic day in January. It was the 20th, to be exact, the day of President Barack Obama’s inauguration, Emotions were running high in the house – history was being made, after all – so at first I didn’t pause when I heard excited shouts in the kitchen. But I had to check out the commotion when my seven-year-old son shrieked: “Mama, come quick! They are going to die!”

I rushed into the kitchen to find young Anton and my husband gesturing frantically at the window. There at the snowy bird feeder clung two green birds, which were decidedly not native. They were the colour of key lime and could have squawked “Polly”. Parrots.

“They are going to freeze!” Anton hollered. “Do something!”

Working on the assumption that these were escaped house pets – I mean, how could a parrot survive outside in snowy New York? – I called the police. The officer who replied said they could only round up stray cats or dogs. “We don’t do parrots,” he said apologetically.

Then my husband, John, remembered that the next-door neighbour once worked as a vet. A dozen cockatiels fly around inside his house, so surely he would know what to do. Informed by telephone that a feathered life was in danger, Michel bolted over the fence in his pyjamas, waving a big net. He moved in close to inspect the birds, pulling on bulky mittens to protect against their talons. Then he checked his Kaufman Field Guide to Birds of North America and let out an amazed shout.

“Myiopsitta monachus!”

We gave him puzzled looks.

“Monk parakeets – native to South America!”

Michel whistled like a budgerigar to lure the visitors inside. The parakeets shot him annoyed glances and turned their backs. Michel came back inside.

“Definitely feral,” he concluded.

The exotic visitors stuck around for weeks, creating wonder to no end. We were thrilled to see such colourful creatures this far north of the Amazon. Their green plumage looked particularly fetching juxtaposed with the blue jays and cardinals – not that we saw the three together often, as the monks didn’t like to mix. However, our social life improved immensely when word got out about the newcomers. Everyone wanted to see this ornithological oddity.

Yet, the glow dimmed as I began to worry about the natural order of things. The intruders scared off the local breeds. They pushed the doves off their perch on the ailanthus tree. The cardinals that once nested so prolifically in our crabapple tree fled. Even city pigeons stayed away. Meanwhile, neighbours whispered that monks had destroyed crops in Argentina. Our flowerbeds could be next. “Give Anton a slingshot,” urged one anxious woman.

I went online and discovered that these pretty animals were in fact destructive and vocal pests. Aggressive colonisers, they would overrun entire neighbourhoods like greedy real estate developers, building the nest equivalent of condominiums to house giant extended families. We had to prevent them from becoming too comfortable. But what would work?

This set off a heated family debate. Anton wanted to cage them, John favoured leaving them free in the garden to let “nature take its course” and I suggested bagging them and throwing them into the Hudson River. At a loss, I called the curator of birds at the American Museum of Natural History, Joel Cracraft. How to get rid of the parakeets?

He paused for a long time before finally answering. “I don’t know.”

He then explained how he thought the monks had got there in the first place. Although each backyard was “idiosyncratic”, an increase in the earth’s average surface temperature – documented by leading climate scientists and possibly caused by greenhouse gas generated from human activity – had an impact here like everywhere else on the planet. Global warming was expanding deserts, shrinking rainforests, rising sea levels and melting glaciers. Warmer winters were prompting some species of birds to venture further north, while others were holding off migrating or not leaving at all. That could be why our intruders were “hanging out”, as he put it. Apparently all the collective heat in the monks’ nests help them survive the cold and some had ventured as far north as Chicago.

“Do you have a bird feeder?” Carhart also asked.

“Yes,” I replied proudly. “I give them wholewheat bread and bacon fat – only the best!”

He huffed. Apparently I was to blame, too. “You may be feeding things that are bringing things in. The huge growth in bird feeders allows certain birds to over-winter.”

Chastened, I closed down the bird restaurant but it didn’t make a difference. The green meanies continued to loiter outside our window, squawking with mockery, occasionally flying off and coming back with what ominously looked like twigs for a nest.

Watching them made me think more carefully about the rest of my garden.

While the parakeets threatened to turn the trees into a tropical housing complex, flower life on ground level was in trouble. The hyacinths burst through the soil every time the mercury hit 55°F. The roses went into their fourth bloom of the season. Worse, the daffodils made one premature showing and never returned. Oddly, though, a tropical Peace Lily that normally lived indoors was doing just fine outside.

Again, I sought expert advice, this time from Todd Forrest, vice president of horticulture at the New York Botanical Garden, who listened with the patience of Freud as I wailed about my withering plot.

“The large shift in weather would trouble even the most patient gardener,” he said sympathetically. “We’re seeing bizarre and frustrating swings from overcast early spring to ridiculously cold and ridiculously warm winters.”

The problem went beyond New York, he assured me. All over the world plants were blooming sooner than before. At Henry David Thoreau’s famed Walden Pond, scientists counted at least 30 species of wild plants that were flowering ahead of schedule. Such early arrivals affected not only gardens but also social lives. The Japanese had moved forward cherry flower celebrations to keep up with the plants. Boston’s annual lilac festival had also been pushed ahead a week. Meanwhile, researchers in Europe were examining whether pollinating insects were adjusting their body clocks too. “What will happen if bees’ life cycles don’t change in line with the plants on which they depend?” Forrest fretted.

Turning back to my little piece of the environment, he explained that increasingly warm winter temperatures in the US north-east were perversely causing cold damage for some plants. They actually needed the cold to become dormant and then wake up again. Broadleaf evergreens were particularly sensitive to winter dry spells. “A plant like hydrangea macrophylla won’t bloom well after suffering winter damage,” he said. I nodded, thinking about the withered snowball bush.

Also damaging were long droughts punctuated by deluge, again the result of global climate change. This created runoff rather than the steady moisture that many plants needed to thrive.

Hearing the panic in my voice, Forrest suggested I experiment with varieties from hot places such as Indonesia. He and colleagues had had some success with what they jokingly called their “global warming” garden, which featured species not typically found in this geographic zone. Non-natives adapting nicely to New York’s changing weather conditions included camellias, grape-hollies, crape myrtles, Japanese apricot and even a Himalayan fan palm.

The Botanical staff was also testing out various conifers that normally didn’t thrive in New York City. For the past five years weeping giant sequoias and Lebanon and deodar cedars had done just fine.

Having said that, for a predictably flowering garden I should stick with the hardy favourites: day lilies, grasses, crabapple trees, roses, hostas and ferns.

“You can’t get any more reliable than a forsythia or lilac,” he added.

I looked out the window. That’s all I had. So why didn’t mine bloom?

“The soil fertility could be low,” he mused. “Did you give the roses a good, hard cutback in the late winter?”

“No,” I admitted sheepishly.

“Welcome to the world of gardeners,” my mentor said agreeably. “All we do, and we do it well, is complain about the weather. We find a lot to substantiate our conspiracy theories.”

Inspired by his pep talk, I went forth with pruning sheers and several buckets of Miracle-Gro. At the first hint of spring, three minuscule forsythia blossoms appeared. Much more responsive were the violet weeds, which proliferated.

As they flourished, something else disappeared. On a hot day worthy of the jungle, the parrots vanished – as mysteriously as they had appeared. John had a theory.

“Maybe it got too warm for them,” he mused. “They’ve probably gone north.”

Judith Matloff is the author of ‘Home Girl’ (Random House, $25)