Almost all those arriving came across the Mediterranean or the Aegean Seas, and half were Syrians fleeing the war. Another 20 percent were Afghans, and 7 percent were Iraqis, IOM and the U.N. refugee agency UNHCR said in a joint statement.
People smuggling operations probably accounted for the majority of journeys and likely earned at least $1 billion, Swing said, taking "anywhere from $2,000 to maybe $6,000 depending on how many members of the family and depending on which smuggling ring it is".
IOM estimates people smugglers in Europe have made $10 billion or more since 2000, maybe much more. "They are certainly getting very well paid for their services," Swing said.
Out of a total of 1,005,504 arrivals to Greece, Bulgaria, Italy, Spain, Malta and Cyprus by Dec. 21, the vast majority -- 816,752 -- arrived by sea in Greece, IOM said.
IOM spokesman Joel Millman said it was impossible to forecast how the flow of migrants would evolve in 2016.
"So much is in the balance, the resolution of the Syrian war, and the disposition of the European border protection moves that are being contemplated," he said.
"We never thought it would reach this level. We just hope people are treated with dignity."
The record movement of people into Europe is a symptom of a record level of disruption around the globe, with numbers of refugees and internally displaced people far surpassing 60 million, UNHCR said last week.
Swing said the war in Syria was only one among many causes, including Ebola and Boko Haram in West Africa, an earthquake in Nepal, conflicts in Libya, Yemen, South Sudan, Central African Republic and Afghanistan and Iraq.
"No wonder you have such a large flux coming north. This is an unprecedented scale of simultaneous complex protracted disasters from the western bulge of Africa to the Himalayas, with very few stable, peaceful spots in between."
U.N. refugee chief Antonio Guterres called on Friday for a "massive resettlement" of Syrian and other refugees within Europe, to distribute many hundreds of thousands of people before the continent`s asylum system crumbles.
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