The Sleeping Porch by Ian Wallace

From chapters.ca: On a hot, steamy summer night, young Brando and his parents move out onto the porch to sleep, hoping they might catch a hint of the breeze. They soon fall asleep, but around midnight Brando stirs. And in the flicker of a firefly, the cat from the nearby graveyard lands on his cot... and so begins an amazing adventure. Together, Brando and the Graveyard Cat soar over the city that is melting in the heat and leap into the night sky, as high as the rings of Saturn. They head north where the arctic seas are full of seals and whales and shimmering icebergs. They scale jagged cliffs and slide down whistling slopes. But when the iceberg they're riding on begins to melt and they both nearly fall into the frigid water, their cries fill the hot summer night. When Brando wakes up the next morning, it all seems to have been a dream... or was it?