Climate change threatens many species by a poleward/upward movement of their thermal niche. While we know that faster movement has stronger impacts, little is known on how fluctuations of niche movement affect population outcomes. Environmental fluctuations often affect populations negatively, but theory and experiments have revealed some positive effects. We study how fluctuations around the average speed of the niche impact a species' persistence, abundance and realized niche width under climate change. We find that the outcome depends on how fluctuations manifest and what the relative time scale of population growth and climate fluctuations are. When populations are close to extinction with the average speed, fluctuations around this average accelerate population decline. However, populations not yet close to extinction can increase in abundance and/or realized niche width from such fluctuations. Long‐lived species increase more when their niche size remains constant, short‐lived species increase more when their niche size varies. Environmental fluctuations are often thought to decrease population abundance. Our simulations show that fluctuations around the average speed of climate change can increase population abundance and range size. The effect differs between species depending on whether their life cycle is fast or slow compared to the fluctuations of climate change.