The first global review of their status has found that populations are declining almost everywhere they live, from Alaska and Canada, to Greenland, Scandinavia and Russia.
The iconic deer is vital to indigenous peoples around the circumpolar north.
Yet it is increasingly difficult for the deer to survive in a world warmed by climate change and altered by industrial development, say scientists.
Reindeer and caribou belong to the same species, Rangifer tarandus.
Caribou live in Canada, Alaska and Greenland; while reindeer live in Russia, Norway, Sweden and Finland.
Worldwide, seven sub-species are recognised. Each are genetically, morphologically and behaviourally a little different, though capable of interbreeding with one another.
Reindeer and caribou numbers worldwide: red denotes herds in decline, green indicates those on the increase and dark grey means no data is available. Reindeer and caribou do not range in areas coloured light grey
These differences between sub-species dictate how each is affected by human impacts.
For example, it has been known for a while that populations of woodland caribou in Canada have declined as human disturbance has increased, caused by logging, oil and gas exploration, and road building, says Liv Vors of the University of Alberta in Edmonton, Canada.
But then reports started coming in that the numbers of other herds were also falling.
"When we discovered that many herds of reindeer also were declining we decided to compile a comprehensive survey to see if this indeed was a global pattern," says Vors.
Vors and Mark Boyce at the University of Alberta contacted other researchers and scoured the published literature and government databases for all the information they could find about reindeer and caribou numbers. They compiled data on 58 major herds around the Northern Hemisphere.
The scientists were shocked to discover that 34 of the herds were declining, while no data existed for 16 more. Only eight herds were increasing in number. Many herds had been declining for a decade or more.
"We were surprised at the ubiquity of the decline," says Vors.
"We knew that woodland caribou in North America were in bad shape." There is also some evidence that populations of migratory caribou in the Canadian Arctic have fluctuated in recent history.
But the researchers were surprised at how migratory caribou and reindeer numbers seem to be falling in synchrony across the Northern Hemisphere.
"When we delved into the status of European reindeer herds, we were surprised that so many were declining. We expected them to be in better shape than North America herds because reindeer, namely the semi-domestic herds, are closely managed by humans."
The scale of the problem is shown by a map upon which the researchers plotted their data, which is published in Global Change Biology.
THE SEVEN SUB-SPECIES
R. t. tarandus. Semi-domestic and wild reindeer that live across northern Scandinavia and Russia. Wild reindeer undertake long, seasonal migrations between summer and winter ranges.
R .t. fennicus. Wild forest reindeer that live in the forests of Finland and the Kola Peninsula of Russia.
R. t. platyrhynchos. Svalbard reindeer that live only on the Spitsbergen Archipelago, which belongs to Norway. Svalbard reindeer have light-coloured fur, and shorter legs than other subspecies.
R. t. granti. Grant's caribou found in Alaska and the Yukon. They reside in large groups and undertake long, seasonal migrations.
R. t. groenlandicus. Migratory barren-ground caribou found across the tundra of Canada and Greenland.
R. t. pearyi. Peary caribou, of which perhaps 700 persist on Canadian high Arctic islands.
R. t. caribou. Woodland caribou residing in the boreal forest, mountains and tundra lowlands of Canada.
"Seeing that sea of red was a sobering moment," Vors says.
"If global climate change and industrial development continue at the current pace, caribou and reindeer populations will continue to decline in abundance," says Vors.
"Currently, climate change is most important for Arctic caribou and reindeer, while anthropogenic landscape change is most important for non-migratory woodland caribou."
For example, climate change is affecting migratory caribou in a number of ways.
Warmer summers mean more insect activity, and caribou and reindeer that are harassed by insects are not able to feed as much to put on weight before winter.
Earlier springs mean plants may be past their prime by the time migrating animals reach their calving grounds, while warmer winters include more freezing rain which can form layers of ice over the ground. The caribou and reindeer cannot dig through the ice to feed, and can then starve en masse.
"In time, however, climate change will become more important for woodland caribou, and landscape change will have a greater effect on arctic caribou and reindeer," Vors continues.
"There likely will be more forest fires in woodland caribou habitat, as well as diseases and parasites transmitted to caribou from white-tailed deer, whose range is expanding northward in Canada. More roads are being built in the Arctic, as well as infrastructures like diamond mines, and these sometimes interfere with migration routes."
Unless something is urgently done, all seven sub-species of Rangifer face a bleak future, says Vors.
"The concern is that their habitat and the climate are changing too quickly for them to adapt."
Fewer than 700 Peary caribou remain
The annual treks of migratory caribou form one of the last remaining large-scale ungulate migrations in the northern hemisphere.
Different sub-species also provide a cornerstone to many indigenous cultures around the circumpolar north, from subsistence hunting of caribou by Aboriginal peoples in Canada, Greenland and Alaska to reindeer husbandry by numerous cultures across Scandinavia and Siberia.
"From a Canadian perspective, the caribou is part of our national identity," says Vors. "Canada's caribou migrations have frequently been identified as one of this country's natural wonders, and the species even appears on our 25-cent coin."