Climate has changed significantly in the Fifth World. With more carbon dioxide in the atmosphere than at any point in the past 50 million years, the climate has changed to more closely resemble the Paleocene-Eocene Thermal Maximum (PETM).
The atmospheric greenhouse effect has become the dominant factor in climate across the globe, overwhelming other concerns like latitude or seasonality. Palm trees grow along the shores of the Arctic and Antarctica, but the temperatures at the poles and the equator differ by only a few degrees. Summer and winter differ by only a few degrees. The entire world has become a jungle.
Civilization’s industrial activity released enormous amounts of carbon dioxide into the atmosphere, which triggered several positive feedback loops, such as the release of large amounts of methane from sources like peat bogs and the deep ocean. In fact, methane has a much greater greenhouse effect than carbon dioxide, so these effects ultimately had an even greater impact on the climate than the carbon dioxide that set them in motion. These positive feedback effects caused climate change to unfold much more quickly than experts had predicted.
The overall effect might have made the earth uninhabitable had other feedback loops not eventually taken hold. When the heat grew so intense that the prevailing cloud regime broke down, a new cloud regime immediately formed. Far more moisture entered the atmosphere, creating much thicker cloud cover, more rain, wetter conditions worldwide, and more intense storms. This strengthened the albedo effect, as white clouds reflected more of the sun's light into space, allowing the rising temperatures to finally begin to stabilize into a new, hotter, wetter equilibrium.
Most places in the Fifth World have a weather pattern common to tropical rainforests, with heavy daily deluges and steady rain falling regularly. They see few sunny days. Most of the time the sky remains overcast at least.
Storms have become more common and more intense. Thunderstorms occur frequently. What we would today consider a very powerful cyclone seems average in the Fifth World. With more uniformly warm water, these storms can range far beyond their old limits. Arctic sailors regularly deal with powerful cyclones. With their increased power, storms can penetrate much further inland before breaking apart. Powerful storms sometimes even maintain their eyes over mountain ranges.
The thick forest canopy helps keep people in the Fifth World out of typical daily rain (and sometimes even completely dry). Many people prioritize how they deal with flooding in their architecture, building houses on stands or even living in trees houses (or whole tree villages) to stay above the water. Others prioritize simple shelters that won't hurt anyone, and that they can easily rebuild, should they collapse in a storm.